How do I make a song a ringtone on my iPhone 4?

[KaylaBartram]

I've been googling how to make a song a ringtone on my iPhone 4, and I get the directions to go to iTunes, right click, get info, options, and so forth. But when I get to the part where I change the start time and end time, it won't let me do anything. I can't click on it, I can't do anything with it..
I bought the song straight from iTunes.
Am I to go about this a different way or is there another way to do it?
Thanks!