How to set a sound clip as my ringtone?

I just got an iphone 4s and I have a sound clip It's just an mp3 but is there anyway I can make it into a ringtone? I want it to be my text notification. Help Please!

I've tryed everything I know how. I have a PC COMPUTER not a Mac!

The sound clip is saved on my computer as well as itunes and is already an mp3 on my phone under music. If that helps

You can drag your sound into iTunes so it will save in your music, then sync it to your phone so you have the sound on your phone. Now download a ringtone maker app and select your sound and click save, follow the steps and you'll have a personalized ringtone.

Use an online converter. Just google "mp3 to mp4 online converter" and there should be some options on converting that sound clip file to a ringtone file. After a successful convert just drage the mp4 file to your itunes ringtones or to your iphone