>>715356398 (OP)AoE empires 2 is a good one to start with, as it has one of the best campaigns to teach you be the bare basics, and plenty of single player to keep you going.
My personal favorites of Supcom (Supreme commander), and at the moment, I'm having a lot of fun playing BAR (Beyond all reason) on and off with friends since it's basically Total annihilation, which I loved as a kid. Buy I don't recommned BAR to someone just starting out on RTS's since it doesn't really do the best job teaching you the basic stuff, unless you have a few friends willing to take the time to do so.
If you want something a bit different, I enjoyed battle fleet gothic 2. Not really what people think of when they say "RTS", but I count it as one and it has a fun campaign.