One good resource is this very forum
Start off by looking at as many pics of dioramas as possible, as that will give you some idea of what you want to build.
Then, start asking questions. Couple of basic things you need to know;
What to build the base out of
what to use for ground work, and how to 'detail' it
Greenery, as in grass, shrubs, and trees
There are a variety of products available for everything, but you can usually find cheap, everyday materials as substitutes.
If I may suggest, try to pick up the excellent "How to build Dioramas" by Shep Paine.