How to get ringtones on an iPhone?

I got the black and blue iPhone 5c. I have a text ringtone I bought along time ago because I thought I could text on my iPod touch but I never figured it out so I traded it for an iPhone. I love my phone but it's driving me crazy because I want my favorite song by my favorite band as my tone but it says I have to buy it again I already have it saved in my iTunes library for the music. My girlfriend has the iPhone 4 she can just go into her iTunes music library and select a song as a ringtone it won't let me do that on mine. I hate to download the song again as a ringtone when I already have it saved in my music. How can I use it as a ringtone?

You can buy them or make them manually (legally) using itunes, it is super easy

http://www.wikihow.com/Make-Ringtones-for-the-iPhone

Get "zedge" from the app store
icon is a Z with purple background
this app has ringtones and backgrounds.