Hills are among the most basic sort of terrain you need for a wargames table. Here’s some nice advice on building inexpensive cardboard hills.
The Schild en Vriend site has a nice article discussing hills, mountains and cliffs on the wargames tabletop
The 1000 Foot General blog has a good tutorial on building rocky desert hills that are suitable for a wide variety of settings.
Here’s a nice tutorial on making cheap and easy expanding foam hills for your wargames table.