How to set a ringtone from iTunes on y iPhone?

I have just brought a song fro iTunes on my iPhone 4S, I would like to set it as a ringtone but when I go into ringtones to set it the song is not there it's in a music folder.

You will need to sync your iPhone with iTunes so it can transfer the song to your phone!

You have to buy the song as a ring tone. So if you have a song you want but you want it as a ring tone, you have to buy it under 'tones'.It sucks and that's why I don't have an iphone

It's simple.change the caller tune option