I have a ringtone on my computer in itunes. what do I have to do to get this ringtone onto my iphone

From what I remember, there should be a category inside the iTunes sidebar labeled "ringtones" when you connect your iPhone to your computer. You just need to drag the sound file into that category, provided that it is in the right format.

A quick search on YouTube revealed this CNET video that shows how to manage and create ringtones in iTunes:

http://youtu.be/3UsKCsqAI4I

Hope this helps!

If your ringtone transfer is successful, go to Settings > Sounds > Ringtone > right at the top of the list.