I've been trying to figure out how to do this too, but I found some good info
http://www.ninetoez.net/moto/maketones.html
That link has info on creating a ringtone from a mp3. Tells you how to properly compress it ect. The guide was created for the Motorola Vxxx, so it might not work for you.
And you will need software to transfer it to your phone, but not every phone uses the same software. For motorola phones, you can use a program called P2Kmanager I think.
in my case, i coverted it to wav, then i used nero wav editor to make it mono and crappy quality.. and then i could load it onto my phone with a java loader app.. search for your specific phone.. i am assuming you have a cable to plug it into your computer..