Starting off with something turn based might be easier, like a simplified battleIsle, or Advancewars, with static unit graphics(to start with) moving over a grid. Start off by creating two units (identical for both sides to start with), one fast, with light weapons, the other slow, with a heavy weapon- and build up unit types after these two are working. You could even make it two player to start with, so you get the game balance right before worrying about AI (which is a bitch, you have to calculate view distance,LOS,waypoints, attack priorities etc). Start off with what I have mentioned above, and you can work your way up to a full RTS.