Anyone Have Apple Iphone? Help Please?

i recently purchased the iphone on contract and need help
i know how to add music by syncing it through itunes onto my iphone and i can listen to my music on my phone ect but how do i put a song as ringtone when i go on itunes and click on device and the top tabs that say summary, info ringtones ect... well when i click on ringtones no ringtones show up i click he all ringtones and selected ringtones but still nothing please help as i would like to change my ringtone to one of my songs as the standard ones are boring

thanks guys!