You're going to get a whole bunch of answers probably better than mine, but I'll stick in my 1 1/2 cents:
. First, what will the dio represent/communicate What type of action or
activity? Where is this happening (urban, field, etc)? What's the weather
like (snow, mud, etc)?
. What will be the "focus" of the dio? You need to determine this so that
you don't overload on peripherals that take away from the main theme.
. After determining this, obviously get the necessary kit(s).
. If this will be historically based, get as much reference material as you
can.
. One method I use is a "dirt box" to set up my figs, vehicles, etc. and get
an idea of how everything will look before I start on the actual base. You
can use 3X5" index cards for structures, toothpicks for trees, stuff like
that. This is particularly helpful in posing figs in certain positions.
You can get much more info by checking the features and forums, and like I said, there are a lot of talented people here who will help any way they can. Anyway, keep us posted on your progress and don't hesitate to ask questions-we all do.
Art