How to set a song from my library as my ringtone?

So I just got an iPhone 5c for Christmas and I was wondering if there was any way I could set a song from my library of songs I've bought from iTunes as my ringtone? And if it's possible, how? My mom did it once but she doesn't remember how. And I'd rather not use my computer but if I have to I will.

Heres a video on how to do it for you