[eluser]Randy Casburn[/eluser]
Without providing more information you're making it difficult to help. Can't read your mind, but I'll take a stab at helping anyway.
Ordinarily your ajax requests simply call separate methods in your controller depending on what you need in your page.
fillform() -> fill out the rest of the form fields
validateform() -> validate the submission
senderror() -> return error notices back to form
etc.
All this stuff presumably would gather data from Models or send back information via JSON encoded strings that is simply echo directly from Models or your Controller.
Hope this helps.