I was pretty fucking terrible at it, too. I began with Brood War, and holy balls I sucked. I'm talking bronze league suck.
Then I played with some dude who handed my ass down to me, and learned. You just need to get rid of bad habits, like building shit you don't really need. Once you have the basics completely down, you get into micro and macro management. I'm pretty decent at macro, but micro not so much. After that, it starts going down to your multi tasking capabilities, and that shit is just hard. You explore/scout enemy positions/tech/army while building your main base and probably expanding, while also building your military accordingly to counter the enemy, and don't forget to do your tech research. The skill ceiling in RTS games is retarded high because of the needed multi tasking. Couple said multitasking with the timing I mentioned, and holy balls it's a sport for the mind. On the fly decision making, planning, resource management (both economical and military), high eye-hand coordination. All at the same time.