There is a lot going on in an RTS game, so you have a lot to pick from. But from my point of view it could be split in two. Base management and army management.
In your case I’d pick the one in which you have the most interest.
If it’s base management, start putting down different building, make them spawn unit over time, add some ressource management, etc.
If it’s army management, start making them attack each other, add different types of unit with different abilities.
Whichever you pick, I’d say the important part is start small and make sure you achieve something in a relatively small timeframe so your motivation stay at its peak.