install.packages("extrafont")
library(extrafont)
font_import()
loadfonts(device="win")       #Register fonts for Windows bitmap output
fonts()                       #vector of font family names
##  [1] "Andale Mono"                  "AppleMyungjo"                
##  [3] "Arial Black"                  "Arial"                       
##  [5] "Arial Narrow"                 "Arial Rounded MT Bold"  

library(ggplot2)
data(mtcars)
ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point() +     
  ggtitle("Fuel Efficiency of 32 Cars") +
  xlab("Weight (x1000 lb)") + ylab("Miles per Gallon") +
  theme_bw() +
  theme(text=element_text(family="Times New Roman", face="bold", size=12)) #Times New Roman, 12pt, Bold
#example taken from the Github project page