#activate the package
library(ggplot2)

#load the dataframe
ID <- 1:400
values <- c(rnorm(200, mean=50, sd=22), rnorm(200, mean=20, sd=10))
class <- c(rep("first", 200), rep("second",200))
my.dataframe <- data.frame(ID, values, class)

#boxplot with 2 categories
ggplot(my.dataframe, aes(class, values)) + 
  geom_boxplot()

#boxplot with 2 categories, 2 colors and the legend
ggplot(my.dataframe, aes(class, values, fill=class)) + 
  geom_boxplot()