Well, to start with you might want to make the landscape. If you want a terrain you could try advanced terrain or BlitzTerrain.
From there you could go on to adding camera movement around the terrain.
You will need variables to hold the amount of each resource which you have, and you will need a GUI to display this information to the user.
For units, you might want to make a base class for all types of unit. This would hold information like position on the terrain, direction the unit is facing, health, which player it belongs to, etc.
For each type of unit you would inherit from this base class adding the necessary functionality as you go.
For buildings, you would do pretty much the same thing, but with a building base class instead.
You might have the classes load in any 3d objects they need in the constructor, and delete them in the destructor. You will need a global list of all units so that you can loop through them and update them, and one for buildings too. You could have the base classes automatically add a pointer to themselves into the relevent list in the constructor, and remove it in the destructor.
The user will need a way to select units. Box selection can be done by using 'dbObjectScreenX/Y' to detect if a unit is inside the box. Single object selection can be done using 'dbPickObject'.
If you want units to be able to navigate you will need to implement some sort of pathfinding.
Do all that and you have the bare bones of an RTS.
[b]
