i would like to have one of my favorate songs as a ring tone on my i phone 4s. is that possible?

A ringtone can be only 30 seconds or so in length, and there is no conversion utility on the iPhone to convert a song to a ringtone including the required length and allowing you to select which portion of the song to be used as the ringtone.

There are a number of utilities available for your computer to convert a song that does not include DRM protection to a ringtone allowing you to choose which portion of the song to be used as the ringtone followed by transferring the ringtone to your iPhone via the iTunes sync process.

Check the App Store. There are ringtone making apps. I downloaded one and used it to create a ringtone from a song in my iTunes. It is a process that requires a computer and iTunes, but it works.