Audio file to iPhone ringtone?

On my PC, I have an audio file short cut on my desktop with a short tune that I would like to use as a text tone on my iPhone 4. I have successfully transferred the audio file to my iTunes library under my music. My question being, once the audio file is on my music on my iPhone and iTunes library, how do I make this my text tone?

I don't think that is enough. I always use Syncios to make ringtone. Because you can choose to convert audio file to SMS ringtone with it.

I thought the ringtone should be different from normal audio file. Isn't it?

Anyway, you should try Syncios, it has many tools, find 'Tools" > "Ringtone Maker", there it is.
You can get Syncios from http://www.syncios.com/