Dinamic dropdown list menu

I have a essays website: http://www.qreferat.com and on top of pages i have a category menu.

I want to creat a dropdown list with all categories in it.
How can i creat a dinamic dropdown list menu?
You can see different categories on: http://www.qreferat.com/lista-referate/romana/index8.php

I need some code suggestions to run fast.

Thnaks for you effort to reply.