Depends what mobile you have. Find out if it can use MP3 or WAV ringtones then get a program like dbpoweramp (www.dbpoweramp.com), rip the cd to whatever your phone supports then you may need to cut it up to get the parts you like. I use cooledit for this but it isn't free and i'm not aware of any good free ones. Once you have your clip, transfer it to your phone via bluetooth or cable and you're done.
If, however, your phone does not support mp3 or wav ringtones and it only does polyphonic (.mid) files. You're out of luck, you cannot convert music to mid. use a site like myt610.com for some decent free mids then transfer them to phone via bluetooth, cable or wap.