I will also second Horacio on Hirst Arts. really cool stuff!
If you're dead-set on making your own, use foam board to make hills. You can find it in arts & crafts stores, in different thicknesses. Don't try to spray-paint styrofoam, though, it will disintegrate. If you cut the foam to use as hills, seal the open edges with spackle before spraypainting.
Then visit a model train store and stock up on little plastic trees and green flock.
When my D&D campaign was going full-steam, I made a new terrain piece for every game...for ten months.
It worked out great, everyone had fun playing on them, and now I've seriously expanded my terrain selection for both wargames and RPGs.
Have fun, and good luck!