There are alot companies that make modeling material for our needs but woodland scenics seems to have a real good package of stuff including videos books and instructional kits.
That being said there are many different ways to create mountians, to eacu his own, cardboard strips woven together w./plaster soaked paper towels, foam blocks glued and texture, premade foam tunnels, and insulating foam that comes in a spray can are a couple of methods that come to mind.
You would probably do better coming up w./ specific questions on what you would like do at that particular time. Good luck , Pat.