on change of pulldown another pull down changes its options available, no refres

[newbeee]

i wish to have a basic form with a few fields to fill out, two of these are for the category and the topic

 

if the category pulldown is changed the topic options change for the category chosen.

but the page should not refresh as the other info will be lost.

 

is ajax the way to do this if so is there a basic template someone can show me that i can change to fit in with what is needed.