Hey Hardhat! Welcome to the forum and the wonderful world of dioramas! If you'll read the previous posts on this thread (and other threads in the Diorama subject) you will gather some more info on how to start! But here are few tips (these are the steps that I use, others most likely have another process):
1) Get inspired--find a pic, cartoon, story, have a creative thought or idea about the subject you want to model.
2) Plan--sketch, write out or somehow develop the idea and plan your dio. Think about the who, what, where, when and why of your dio and the idea you want to convey to your audience (those who look at the dio). Ask your friends and/or other modelers opinions--maybe show it to them and don't explain it to see if it the idea makes sense.
3) Get the model(s)--buy or pull the kit(s) from the shelf and start working. Modify kits as needed (battle damage, figure posing, turned wheels, etc) and continue to think about and plan your idea--sometimes this is an ongoing process.
4) Layout your dio once you have some progress made on your kit(s). Determine how big a base you will need, how you want to position the kits within the scene.
5) Groundwork--I use styrofoam blocks and carve them to shape, cover them with plaster cloth (Woodland Scenics C1203), I also use Drywall Joint Compound for minor touch-ups or build-ups on the terrain. When it is still damp, add any vehicle tire or tread marks, footprints, etc. Once dry, I brush the groundwork with white glue and sprinkle on sand and the gritty stuff that builds up in the street, curbs and gutters (sift it first to get out the big stuff and trash). Once that's dry, I paint the earth color, add any buildings. Brush on more white glue followed by a sprinkling of static grass or snow (Woodland Scenics products), add any rubble from buildings here. Add trees and bushes (if you want to know more, ask) then spray with Testor's Dullcoat in the spray can.
6) Add kit(s).
As far as a wire fence, depends on the scale you are working in and the type of fence you are going for: barbed or straight wire/farm fence? There are aftermarket (AM) photoetch (PE) kits for barbed wire in various scales or you can make your own. For farm fence, just find some wire with a gauge that is suitable for the scale you are building, find a good reference (real fence or internet pic) and start working!
Good luck and keep us posted!