Be glad to help.. But first.. If you can swing it, pick up a copy of How to Build Dioramas, by Shepard Paine. You can find it on Amazon.com and whether it's the first or second edition doesn't matter.. Both printings are chock-full of "everything you need to know but were afraid to ask".. I'm serious, Vector... If you only ever buy ONE modeling book, this one's the Bible and will cover much more than we can type in here...
Shep's book will start you at the beginning, from planning and sizing, and take you through groundwork, materials, topography, painting & weathering, figure painting and conversions, structures, foliage, shadow boxes, electrical wiring, and photographing...
I bought my copy back in the early 80's and still use it today... However, if you want to skip it and take the plunge, you can, although I highly reccomend that you don't...
That being said, the first thing you need to do is start planning.. You need to decide exactly who, what, where & when before you start anything else, what story you're trying to tell... Gather the kit(s) your going to use and a sheet of paper to use as your "base" to get an idea of the size you'll need.. Use crumple up tissues for any low foliage, some paper-tube "trees" (if your going urban, then forget that last part)ing draw in any roadways, intersections, sidewalks, build "foot-prints", and such... Get all that stuff laid out and mocked up...
Here's a shot of a current one I'm doing that's in the planning stage. (The American truck is a "stand-in" for an Opel Blitz I have yet to start).. Note the drawings of the RR tracks and road..
For the mock-up, you're going to want to use your kit(s) and should have them fairly complete when you start the mock-up.. Use various-sized boxes for buildings and such...
That should be just information to get you started.. Feel free to ask anytime you need help, all of us are happy to oblige...