How to set a ringtone from a song in your library to your iPhone 4?

I just can't figure it out

If you want a song as your ringtone, you'll have to buy the ringtone from the iTunes Store.

There are also Apps in the App Store that let you create Ringtones from your Songs.

First of all just connect your iPhone with iTunes on your PC and then just follow-up the steps given here: http://www.pcadvisor.co.uk/how-to/audio/3274143/make-song-your-iphone-ringtone/… This will surely work. Worked for me. Enjoy with your customized ring-tone. Hope you find this useful.