Hi Deano.
Best plan would be to have a picture in your head of what you want it to be. There are more than Just
Airfix or
Tamiya to choose from so dont let that limit you.
Do you have a specific image as to what you want. WWII, Modern. Desert, Europe, summer, winter.
Which scale do you WANT to build in.
Next step is how big you can go. Do you have room. A dio in 1/72 can be quite small and still have room for a good few things. The same in 1/35 will be huge!
Once you have the scene, and the scale, you can start to find the models. Most diorama have a centre piece. The most important part/model. Choose that and then find things that would fit in with it. I am sure there are more than enough on here that could help with that.
Once you have found the actors for the dio, you can start searching the interweb. Say you wanted a British tank as the central 'focal point', you know where you want it to be; Battle ground and the scale. Google is your friend! So an Image search for
Crusader tank. 1/35 North Africa. Will bring up hundreds of images of models. You can see that there are models in the scale that you can then 'look into' by adding
review to the search You will find a lot of links to hobby sites that have seen the model and said, or shown, what it is like.
( I now want the new Border Models Crusader MkIII ! lol ).
Having found the star of the show, you can start to find the supporting cast. Figures, Trucks, Towed cannon and the such.