There are hundreds of videos on making all kinds of scenery for wargaming and model railways - and lots of different ways of doing it. Many of the sites promote specific manufacturer's products, but they will give you some useful ideas. Try 'The Terrain Tutor' - his videos are more watchable than some.
I've used the stuff you buy for tropical fish tanks - but don't buy it from your local pet store!
This is about a quarter of what I got last year direct from China for about £40:
A lot of these are made from several pieces which can be taken apart or added to. I used some of them for the bushes and shrubs here (The figures are 28mm (1/56) scale from the Wargames Factory):
You can also use plants from your garden. These will need to be preserved though if you want to keep the dio for any length of time.
I did this one about two years ago. All the plants are from my garden and were just sprayed with matt varnish. They've kept most of their colour (The figures and vehicles are from the
Airfix Robogear range which I think is also 28mm scale):