This is a simple (or so I thought) use of AJAX, but i can't seem to figure it out...

I want to add a button to my page that when clicked, it runs a query in MySQL, then returns the value (a random value) to the form field.

This seems like an easy application of AJAX, but I can't find anything that discusses it, and I am not a highly skilled developer... Any thoughts or suggestions or places I can look?