Other than converting your Mp3 songs to AAC version through itunes (using desktop or iMac), there is another ONE way to have your own ringtone using, Garageband. Hewhew.
First, make sure you have Mp3/songs that intended to be used as the ringtone, saved in your Music folder. For this you got to read another tutorial.
Install Garageband application using appstore.

Next you may refer to the following pictures. Easy peasy.