Try:
df <- data.frame(cbind(x = c("C","G","D","A","R"), y=rnorm(5)), stringsAsFactors = FALSE)
head(df)
df$x <- factor(df$x, levels = c("C","G","D","A","R"))
levels(df$x)
class(df$y)
df$y <- as.numeric(df$y)
ggplot(df, aes(x = x, y = y)) + geom_bar(stat = "identity")