Im trying to populate a combobox based on the value of another combobox

trying to do it in javascript at the moment but no luck

combobox 1 is called "group" and has values a,b,c,d,h and v
when one of these is selected cobobox 2 called "car" needs to be populated with the relevant information

a = ford ka
b = ford focus

and so on

can anyone help ?

Scott