If you don't understand RTS's begin by understanding that they're really hard.
Playing another human being is a matter of being able to concentrate on more than one thing at once, thinking ahead, and thinking at lightning speed. I have played a few people on Battle.net and I, having beaten the campaign (something I never accomplished in Starcraft 1) was surprised at how bad I was.
The secret is, what seems to me, to be: better safe than sorry, but better fast than slow. Always give in to the desire to make another SCV. But also always crank out those zerglings early, and post those cannons everywhere. Always be on edge and believe that your enemy could conquer you at any moment, and do your best to cover all of your bases, all at the same time.
If you don't know if you're up to it, don't waste your money. I have friends that love it, and some that simply can't get themselves to succeed at them. It's all up to you.