You want reorder(). Here is an example with dummy data
set.seed(42)
df <- data.frame(Category = sample(LETTERS), Count = rpois(26, 6))
require("ggplot2")
p1 <- ggplot(df, aes(x = Category, y = Count)) +
geom_bar(stat = "identity")
p2 <- ggplot(df, aes(x = reorder(Category, -Count), y = Count)) +
geom_bar(stat = "identity")
require("gridExtra")
grid.arrange(arrangeGrob(p1, p2))
Giving:
Use reorder(Category, Count) to have Category ordered from low-high.