If you have a Mac, you can use Garageband to edit a song down to 30 seconds or so, then use Export->to iTunes menu item, and enable it to be synced to your iPhone in iTunes.
If you use Windows, there are some apps you can buy for a few pounds, but I'm not sure of any names. A Google search may help.
i had this probem, best thing is a website called audikon you can get pretty much anything its free and easy to use just download, click save to and choose where you want to save it, then go into itunes and add the file to your library and make sure the ringtone icon is showing