How do I create a realistic miniature wargaming terrain scene?

I have a bunch of Gundam (i.e robot, think Transformers but without the transforming) minifigs and I want to recreate some of the scenes from the movie.

One scene involves a quiet town with some forests around it.
Another scene involves a desert area in the Middle East.
A third scene involves a jungle in South America (think: Amazon)
A fourth scene is set in space, with some asteroids floating around

I’m not sure which of the scenes would be feasible for me to create the terrain for. I know that stores like Games Workshop sell premade terrain, but I prefer to make my own. It doesn’t have to be too realistic (i.e. I don’t really care if the trees have little detailed branches or if the asteroids have perfectly shaped craters).

I am hoping that someone with experience in miniature wargaming could help me with this, but any advice you can give me would be appreciated.