You don't have much information, so I'm going to assume you have an Apple computer. If that's the case, don't waste your money buying ringtones. Instead buy a DRM-free version of the song you want from someplace like Amazon. (all of Amazon's mp3's are DRM free, so you really should never buy anything with DRM ever again!) Once you have the song, that you can now use with whatever you want, open it in Garageband and select a 40 second clip. Then go to "share" and export it to iTunes as a ringtone.
It's that easy and you're not locked down to annoying DRM. If you don't have an Apple computer, there are other ways to convert DRM-free mp3's to a ringtone for your iPhone. It's a little more difficult, at first, but definitely worth it in my book. Just do some Googling and you'll find it in no time.