I have not used ajax yet but I see a real reason to use it now and was wondering if someone can help me with htis

i have a for

I have a textbox called lastname and i have a checkbox called sibling -- now when this is checked I want a popup to popup with all the firstnames of children with matchign lastnames (select firstname from child where lastname='lastname')
then I want to pick a lastname (or select none) and this will populate 3 more fields called contact1 contact2 and contact3 with the info from the database for the child chosen.

can someone help start me on this -- at least the ajax part?