Practice with a few programs. One I like is called 'Bryce'.. THen tweak your results in photoshop. Also, I don't like it, but try Terragen.
Well, ive used terragen, pretty cool.. The bryce never used, i tried 3d max, but thats just confusing haha
I use Bryce all the time. I'll post my gallery in a new thread. Its got all of my work with bryce and photoshop in it.