You need styrofoam, I just saved up a bunch at work from shipping boxes as they came in, but you can either buy it at a hardware store(kind of a waste of money they are pretty expensive) or you can go to stores and ask for any extra they have. Then you need grout which is the most commonly used thing for these, whatever color you want. If you want a different color of rock than the grout youll need the extra color mix or paint. And you'll need a low temp glue gun and sticks and great stuff to fill in holes, and a non toxic sealer to coat over the grout.
If you look around on here theres a great guide how to make a fake rock in 10 easy steps...tells you everything you need to do and everything you need to buy and has lots of pictures.