all depends
what you\'re aiming for and how high you want to build it up. As a basic tip, try using light-weight materials: bits of foam, plywood, cardboard, even papier mache. Then cover it with a light coating of filler mixed with white glue.
The traditional stuff for rough groundwork is \"spackle\" - plaster filler, sometimes with stuff mixed into it to give texture - but everybody uses their version. Two part car body filler was my favorite for a while but it\'s expensive for big jobs.
For stones etc: a good tip is to use the fine dirt that collects along the edges of roads in the gulleys - it\'s usually around the right scale.
Hope that helps