How to customize ringtone on the iphone 4s?

I just got a iphone 4s.
I sync some songs to the phone from itunes ( i didn't buy them)
When i go to setting>sounds>ringtones, i can't find the customize tab?
Do anyone know how to use a song that you synced in as a ringtone?

I have never used an iPhone but it would not surprise me if you couldn't.