There are many free web sites and apps that let you create ringtones from your songs. We don't allow posting or promotion of 3rd party services here, so I can't recommend any specific app, but if you search the Internet you'll find plenty of articles or video tutorials. I recommend using an app on your phone vs. a web site though, I believe it's easier and quicker.
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Any music software that allows you to "copy and paste"; you will be able to make your own ringtones. If you don't have any software along these lines then go to www.download.com, and you should be able to find a music/ringtone software from there. Once you make the ringtone, just plug in your phone and copy and paste it to what ever file you want in your phone.