That's a tall order, fran_shoes..! So many things are possible. I don't do (many) ships nor water stuff, so I won't be able to give you many tips there, but I do model aicraft and occasionally do a diorama with one of them.
First thing, really, is to think about what you want to do, want to show, want to represent... Basic airfield set up, crash landing, maintenance, fuelling-up, .. Get books, mags, check the Net for pictures of your subject model and see what is possible. There are tons and tons of stuff you can buy to make things the way you want or make your life easier: trees, foliage, jerry-cans, ground vehicles, crew, pilots, available in mainstream manufacturer's catalogues (Hasegawa has 1/72 Japanese and US tanker trucks for instance), from other manufacturers such as Verlinden, or from 'garage' companies...
And of course, there are opportunities to modify existing products or scratchbuild stuff you can't find or do not want to pay lots of money for!
So, check your references and tell us what you have in mind, then we can guide you to the right products and tell you about the right techniques.
Domi