What computer are you using and what OS is it running?

I ask because if you are using a Mac, then you can create RingTones in GarageBand very easily.

If you are running Windows, try this:
Create a folder in My Documents called Ringtones. It does nothing except allow you to find stuff later.
In iTunes, select the track you want and make a note of the start point that you want to use. Get Info on the track and change the start and end points, no more than 30 seconds apart. Click OK.
In iTunes, convert the now short track to AAC (This is important). Drag the AAC version into the Ringtones folder in My Documents. Back in iTunes, select the original track, Get Info and set the track length back to the original.
In My Documents > Ringtones, change the extension of the track you just dragged there from m4a to m4r.
Drag the file into the Ringtones folder of iTunes. If you don't don't have one there yet, one will be created when you drag the newly create ringtone into that top left section.