StarCraft Legacy has a strategy section in the forums.
This is a very old site with helpful people. SCLegacy carefully checked every rumor of a SC2 for years! While remaining a center of sanity, far as I was concerned, in all aspects of the original StarCraft. The other sites mentioned above may have lots more material but SCLegacy is still where I look for wisdom.
Or, just do this. Build some crap. Scout and see what crap the opponent is building. If his crap can't stop your crap, run over and beat on him. But be prepared for his crap to show up first and wreck your water sprinklers! For revenge send your crap over to mess on his carpet.
Change your build order even though it looks like you have better crap and the upper hand and you don't need to. This will rattle your opponent who will change his crap build order purely out of fear! Rinse and repeat until you either win or lose. Or the other player quits in disgust. If you win be condescending. If you lose, be a bad sport about it; otherwise people will think you're weird.
Next time, build other crap different from the crap you built the first time. Don't become predictable. If your crap turns out to be totally useless, it's not your fault! It's because the other player is a noob using trick build outs! And because the stupid developers screwed over your favorite faction in the last balance patch.
Play against the AI a lot. Gives you opportunity to sharpen your unpredictable on the fly changes of strategy and tactics! Watch lots of videos on youtube featuring Koreans because only Koreans truly understand this game. This is because of their genes. Notice how they always win even though nobody can understand why. This is the key.