Make the ringtone on an iPhone 4S a song from your music library?

Can I set the ringtone for my iPhone 4S from something from my music library on my phone? If so, how?

1.select the music in library
2.right click it and choose "get info">"options"--set up "start time" ""end time"
3.click ok
4.right click it again> "create aac"
5. You will see a.m4r music file in the music list
the length of music that can play in iphone is less than 40 seconds.