The way I've done it in iTunes in the past was
1- select the song you want as ringtone
2- Command and letter i for Info
3-click Options tab
4- enable a Start time and End time. Click OK
now
5- Convert the song to aac.
6- drag that converted song to the desktop.
7- change the extension of the song on your desktop to m4r. - this is the Tone extension that iTunes recognizes
8- drag this m4r into iTunes
9- it will now be in the Tones section of iTunes
10- plug in iPhone and setup syncing Tones.
11- go to Sounds on iPhone and select it as ringtone.
Long convoluted process, but it's a free ringtone of your music.