Easiest way to change font family in ggplot is to make use of extrafont package.

library(ggplot2)
library(extrafont)
## Registering fonts with R
loadfonts(quiet = T)

fonts()
##   [1] "ABSALOM"                         "Agency FB"                      
##   [3] "Aharoni"                         "Algerian"                       
##   [5] "Alibi"                           "Andalus"                        
##   [7] "Angsana New"                     "AngsanaUPC"                     
##   [9] "Aparajita"                       "Arabic Typesetting"             
##  [11] "AR BERKLEY"                      "AR BLANCA"                      
##  [13] "AR BONNIE"                       "AR CARTER"                      
##  [15] "AR CENA"                         "AR CHRISTY"                     
##  [17] "AR DARLING"                      "AR DECODE"                      
##  [19] "AR DELANEY"                      "AR DESTINE"                     
##  [21] "AR ESSENCE"                      "AR HERMANN"                     
##  [23] "Arial Black"                     "Arial"                          
##  [25] "Arial Narrow"                    "Arial Rounded MT Bold"          
##  [27] "Arial Unicode MS"                "AR JULIAN"                      
##  [29] "Baskerville Old Face"            "BATAVIA"                        
##  [31] "Bauhaus 93"                      "Bell MT"                        
##  [33] "Belwe Bd BT"                     "Belwe Lt BT"                    
##  [35] "Belwe Cn BT"                     "Berlin Sans FB"                 
##  [37] "Berlin Sans FB Demi"             "Bernard MT Condensed"           
##  [39] "Blackadder ITC"                  "Blade Runner Movie Font"        
##  [41] "Bodoni MT"                       "Bodoni MT Black"                
##  [43] "Bodoni MT Condensed"             "Bodoni MT Poster Compressed"    
##  [45] "Book Antiqua"                    "Bookman Old Style"              
##  [47] "Bookshelf Symbol 7"              "Bradley Hand ITC"               
##  [49] "Britannic Bold"                  "Broadway"                       
##  [51] "Browallia New"                   "BrowalliaUPC"                   
##  [53] "Brush Script MT"                 "Calibri"                        
##  [55] "Calibri Light"                   "Californian FB"                 
##  [57] "Calisto MT"                      "Cambria"                        
##  [59] "Candara"                         "Candy Round BTN"                
##  [61] "Candy Round BTN Cond"            "Candy Round BTN Cond Lt"        
##  [63] "Candy Round BTN Lt"              "CASMIRA"                        
##  [65] "Castellar"                       "Centaur"                        
##  [67] "Century"                         "Century Gothic"                 
##  [69] "Century Schoolbook"              "Chiller"                        
##  [71] "Clarendon Blk BT"                "Clarendon BT"                   
##  [73] "Clarendon Cn BT"                 "Clarendon Hv BT"                
##  [75] "Clarendon Lt BT"                 "Colonna MT"                     
##  [77] "Comic Sans MS"                   "ConcursoItalian BTN"            
##  [79] "ConcursoItalian BTN Wide"        "Consolas"                       
##  [81] "Constantia"                      "Cooper Black"                   
##  [83] "Copperplate Gothic Bold"         "Copperplate Gothic Light"       
##  [85] "Corbel"                          "Cordia New"                     
##  [87] "CordiaUPC"                       "Courier New"                    
##  [89] "Curlz MT"                        "DaunPenh"                       
##  [91] "David"                           "DFKai-SB"                       
##  [93] "DilleniaUPC"                     "DokChampa"                      
##  [95] "Ebrima"                          "Edwardian Script ITC"           
##  [97] "ELEGANCE"                        "Elephant"                       
##  [99] "ELLIS"                           "English157 BT"                  
## [101] "Engravers MT"                    "Eras Bold ITC"                  
## [103] "Eras Demi ITC"                   "Eras Light ITC"                 
## [105] "Eras Medium ITC"                 "Estrangelo Edessa"              
## [107] "EucrosiaUPC"                     "Euphemia"                       
## [109] "EXCESS"                          "FangSong"                       
## [111] "Felix Titling"                   "Footlight MT Light"             
## [113] "Forte"                           "Franklin Gothic Book"           
## [115] "Franklin Gothic Demi"            "Franklin Gothic Demi Cond"      
## [117] "Franklin Gothic Heavy"           "Franklin Gothic Medium"         
## [119] "Franklin Gothic Medium Cond"     "FrankRuehl"                     
## [121] "Freehand521 BT"                  "Freehand575 BT"                 
## [123] "FreesiaUPC"                      "Freestyle Script"               
## [125] "French Script MT"                "Gabriola"                       
## [127] "Gadugi"                          "Galeforce BTN"                  
## [129] "Garamond"                        "Gautami"                        
## [131] "GENUINE"                         "Georgia"                        
## [133] "Gigi"                            "Gill Sans Ultra Bold"           
## [135] "Gill Sans Ultra Bold Condensed"  "Gill Sans MT"                   
## [137] "Gill Sans MT Condensed"          "Gill Sans MT Ext Condensed Bold"
## [139] "Gisha"                           "GIST-TMOTIlango"                
## [141] "Gloucester MT Extra Condensed"   "Goudy Old Style"                
## [143] "Goudy Stout"                     "GrilledCheese BTN Cn"           
## [145] "GrilledCheese BTN Toasted"       "GrilledCheese BTN Wide Blk"     
## [147] "Haettenschweiler"                "Harlow Solid Italic"            
## [149] "Harrington"                      "HELTERSKELTER"                  
## [151] "HERMAN"                          "High Tower Text"                
## [153] "Hot Mustard BTN"                 "Hot Mustard BTN Poster"         
## [155] "IMG Baby"                        "IMG Love"                       
## [157] "IMG Seasons"                     "IMG Symbols"                    
## [159] "IMG Travel"                      "Impact"                         
## [161] "Imprint MT Shadow"               "Incised901 BdCn BT"             
## [163] "Incised901 Ct BT"                "Incised901 Nd BT"               
## [165] "Incised901 NdIt BT"              "Informal Roman"                 
## [167] "IrisUPC"                         "ISABELLE"                       
## [169] "Iskoola Pota"                    "JasmineUPC"                     
## [171] "Joan"                            "Jokerman"                       
## [173] "Juice ITC"                       "JUSTICE"                        
## [175] "KaiTi"                           "Kalinga"                        
## [177] "Kartika"                         "Khmer UI"                       
## [179] "KodchiangUPC"                    "Kokila"                         
## [181] "Kristen ITC"                     "Kunstler Script"                
## [183] "Lao UI"                          "Latha"                          
## [185] "Wide Latin"                      "Leelawadee"                     
## [187] "Levenim MT"                      "LilyUPC"                        
## [189] "Lucida Bright"                   "Lucida Calligraphy"             
## [191] "Lucida Console"                  "Lucida Fax"                     
## [193] "Lucida Handwriting"              "Lucida Sans"                    
## [195] "Lucida Sans Typewriter"          "Lucida Sans Unicode"            
## [197] "Magneto"                         "Maiandra GD"                    
## [199] "Malgun Gothic"                   "MANDELA"                        
## [201] "Mangal"                          "Marlett"                        
## [203] "Marquisette BTN"                 "Marquisette BTN Light"          
## [205] "Marquisette BTN Lined"           "MATTEROFFACT"                   
## [207] "Matura MT Script Capitals"       "Microdot"                       
## [209] "Microsoft Yi Baiti"              "Microsoft Himalaya"             
## [211] "Microsoft JhengHei"              "Microsoft New Tai Lue"          
## [213] "Microsoft PhagsPa"               "Microsoft Sans Serif"           
## [215] "Microsoft Tai Le"                "Microsoft Uighur"               
## [217] "Microsoft YaHei"                 "Miriam"                         
## [219] "Miriam Fixed"                    "Mistral"                        
## [221] "Modern No. 20"                   "Mongolian Baiti"                
## [223] "Monotype Corsiva"                "MoolBoran"                      
## [225] "MS Outlook"                      "MS Reference Sans Serif"        
## [227] "MS Reference Specialty"          "MT Extra"                       
## [229] "MV Boli"                         "Myriad Web Pro"                 
## [231] "Myriad Web Pro Condensed"        "Narkisim"                       
## [233] "NATURALBORN"                     "NEOLITH"                        
## [235] "Niagara Engraved"                "Niagara Solid"                  
## [237] "Nightclub BTN"                   "Nightclub BTN Cn"               
## [239] "Nightclub BTN UltraCn"           "Nirmala UI"                     
## [241] "Nyala"                           "OCR A Extended"                 
## [243] "Old English Text MT"             "Onyx"                           
## [245] "OPENCLASSIC"                     "Palace Script MT"               
## [247] "Palatino Linotype"               "Papyrus"                        
## [249] "Parchment"                       "Perpetua"                       
## [251] "Perpetua Titling MT"             "Persia BT"                      
## [253] "Plantagenet Cherokee"            "Playbill"                       
## [255] "Poor Richard"                    "PRETEXT"                        
## [257] "PrimaSans BT"                    "Pristina"                       
## [259] "PUPPYLIKE"                       "Raavi"                          
## [261] "RADAGUND"                        "Rage Italic"                    
## [263] "Ravie"                           "Realvirtue"                     
## [265] "Rockwell"                        "Rockwell Condensed"             
## [267] "Rockwell Extra Bold"             "Rod"                            
## [269] "SaiEmbed"                        "SaiIndira"                      
## [271] "SaiMeera"                        "SaiSai"                         
## [273] "SaiVrishin"                      "Sakkal Majalla"                 
## [275] "Script MT Bold"                  "Segoe Print"                    
## [277] "Segoe Script"                    "Segoe UI"                       
## [279] "Segoe UI Light"                  "Segoe UI Semibold"              
## [281] "Segoe UI Semilight"              "Segoe UI Symbol"                
## [283] "SHELMAN"                         "Shonar Bangla"                  
## [285] "Showcard Gothic"                 "Shruti"                         
## [287] "SimHei"                          "Simplified Arabic"              
## [289] "Simplified Arabic Fixed"         "SimSun-ExtB"                    
## [291] "Snap ITC"                        "Sneakerhead BTN"                
## [293] "Sneakerhead BTN Condensed"       "Sneakerhead BTN Outline"        
## [295] "Sneakerhead BTN Shadow"          "Sonic XBd BT"                   
## [297] "SonicCutThru Hv BT"              "Space Bd BT"                    
## [299] "Starliner BTN"                   "Stencil"                        
## [301] "Swiss921 BT"                     "Sylfaen"                        
## [303] "Symbol"                          "TABUni-Tamil021"                
## [305] "Tahoma"                          "TAMUni-Tamil150"                
## [307] "Tempus Sans ITC"                 "Times New Roman"                
## [309] "Traditional Arabic"              "Trebuchet MS"                   
## [311] "TRENDY"                          "TSCu_SaiIndira"                 
## [313] "Tunga"                           "Tw Cen MT"                      
## [315] "Tw Cen MT Condensed"             "Tw Cen MT Condensed Extra Bold" 
## [317] "Uni Ila.Sundaram-01"             "Uni Ila.Sundaram-02"            
## [319] "Uni Ila.Sundaram-03"             "Uni Ila.Sundaram-04"            
## [321] "Uni Ila.Sundaram-05"             "Uni Ila.Sundaram-06"            
## [323] "Uni Ila.Sundaram-07"             "Uni Ila.Sundaram-08"            
## [325] "Uni Ila.Sundaram-09"             "Uni Ila.Sundaram-10"            
## [327] "Uni-Tamil150"                    "Utsaah"                         
## [329] "VAGRounded BT"                   "Vani"                           
## [331] "Verdana"                         "Vijaya"                         
## [333] "Viner Hand ITC"                  "Vivaldi"                        
## [335] "Vladimir Script"                 "Vrinda"                         
## [337] "Webdings"                        "Wingdings"                      
## [339] "Wingdings 2"                     "Wingdings 3"
df <-
  data.frame(
    key = c("Compliance", "Information Technology"),
    budget = c(25000, 15000)
  )

ggplot(data = df) +
  geom_bar(aes(x=key, y = budget), stat = "identity") + 
  labs(x = "Department",y = "Budget in $", title = "Budget by Department", subtitle = "Budgeted Amount Distribution for XXX", caption = Sys.Date()) +
  theme_gray(base_family = "Arial Black")

A custom theme can also be created to set fonts for different elements in the graph.