You just need the following programs installed...
-iTunes (you should have that anyway) download
-Qualcomm PureVoice 3.1 for Windows download
Itunes>Settings>Advanced>importing>
Change the setting to wav 8-bit, Mono.
Then you can use itunes to convert the mp3 to an 8-bit, Mono, wav file by right clicking it.
The Qualcomm software is a little harder to use. Follow their instructions to convert the wav file to a QCP file. This QCP file is a valid ringtone format for many phones. Just move it to the phone somehow (I use my TransFlash memory card) and select it as your ringtone for one contact or for all.
Don't pay for ringtones when you can make them yourself. Comment if you have any questions.