Need help with Combo Box

[Toffee]

Newbie question, I want to code a 2 levels combo box, with the
following behavior:

1. The 1st box to display say the countries, and the 2 box to display
provinces/states.

2. Initially, only show the 1st box and hide the 2nd box.

3. If the user picks:
Canada, then the 2nd box shows, while staying on the same page.

4, If the use picks:
USA, then jump to a different page.

How do I do that?

Thanks