How to use an mp3 file as a ringtone on my iPhone?

Hello,

First off, I recognize that I may be biting off far more then I can chew.
I want to be able to somehow load a MP3 file on to my iPhone 4S and then make it a ringtone.
Can someone please tell me what I need to do in order to make this happen?
My task is being hampered by the simple fact that my current PC is unable to access either iTunes or iCloud because I'm not able to change the browser to one the these sites require. (Yes, it is a shared computer ... for now).
Thank you everyone for your time in reading this and for any and all assistance.