First of all: good luck!

But as a RTS fan, I can tell you it is more difficult than making a simple shooter, where you mostly have to deal with the level design (enemy behaviour is generally not too complex).

For example, what AI system do you plan to use for dynamic pathfinding and decision making? It normally requires good map decomposition into usable data structures for managing buildings, developing technologies, and strategic and battle army/unit command.

So first start with a simple game, and you will see what you need for further developments...