Help with ajax

Help with ajax

Hi.... I really have no clue how to get this done.... I have a form with a few textboxes and a select box as well.... I want to have the form fill itself if you happen to select one of the options of the select box.... the data is at a mysql database.... I know this is probably fairly complicated so you dont need to explain it step by step, I would just appreciate a link to an example of this being done.... thanks in advance

Just in case you want more backgorund info....
I have name, last name, ID, Telephone number and address textboxes
I also have the select box with a bunch of IDs of users that have already been registered so I want it so that if I click on one of the option IDs, it will pull up the data and autofill the form (making it non-modifiable)