CMS Made Simple Forums

Hi guys,
I'm a newb in CMSms, and I have a big task on my head,
I want to create some sort of hierarchy.
For that I need some help, I wan to be able to give users on sign-up different options that are dependant on their previous choices
let me explain  :
A user resgisters, he has a select box, with these options:
-GMC
-lamburgini
-fiat

I want to be able to give the user a select box with specific manufacturer models
for instanse if he chose fiat
the second select box will only populate :
-panda
-uno
-punto

and if he chooses lamburgini, some other elements
and I HAVE to add them all dynamically

I'm stumped, and I would love some advice on this, or maybe just a point in the right direction

thanx in advance