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gorby
Now, that's a BIG question.
First you'll need a board of some kind. Some people use wood and some others use a picture frame (usually loads in charity shops at good prices). I usually use insulation board. The benefit of using insulation board is that it's easily carved and usually available free in a skip near you. 🙂
Then I often cover it in soil. You can buy diorama soil or alternately, real soil can be used as long as it's heat treated so to kill micro organisms. Mixed with diluted PVA glue it can be applied as a (very messy) paste to the surface which dries rock hard.
If you intend to use static grass it looks a lot more realistic if you use two or three different colours and lengths.
There are loads of items which can be bought or scratch-built to make dioramas more realistic, it all depends on how much work or money you are planning to devote to the project. More detailed advice depends a lot on the type of dio you are intending.

John Hughes
Alternately, there are premade dios which require minimal assembly and painting. They're a good option for a beginner.