Select Get Info
In iTunes, right click or control-click on the song you want to use. Select Get Info.
Set start and stop time
Select the Options Tab. Set the desired strat and stop time. Click OK
Create AAC Version
Again, right click or control-click on the song and choose Create AAC Version. This will create a duplicate of the song.
Show in Finder
Once the conversion is complete. Right click or control-click and select Show in Finder.
Locate duplicate version
In Finder, locate the duplicate song. Note the smaller file size. Drag this to your desktop and remove from iTunes (if you wish).
Change extension to m4r
Change the file extension to m4r then drag it back into iTunes. Choose the Ringtones tab in iTunes to see the new ringtone. Ringtones can’t exceed 30 seconds and must be added to your iPhone using iTunes.