Margins around plot can be modified with theme() and plot.margin= where you provide size of margins starting with top, then right, bottom and left. Library grid is attached to use function unit().
library(grid)
ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point() +
theme(axis.title.x = element_text(family="Times",size=20,
face="bold",colour = "Black",vjust=-1,hjust=0.5))+
theme(plot.margin=unit(c(1,1,1.5,1.2),"cm"))
