I want to change the value of a drop box

I'm new to javascript, so this might seem like a simple task to others.
I made a select menu with HTML, and i would like it to change the values of another dropbox, depending on the value chosen in the first drop box.

For instance, i pick "Schools", in one drop box, which sets the next drop box's values to a list of schools on the change of the first one.

How can i do this?

There are lots of examples on the net, but here is one to get you started:

http://www.quirksmode.org/js/options.html

BaldEagle