Okay first, forget everything ace just said. "I like to go all out with carriers" Yeah... that's a wonderful way to get yourself prepared for human opponents.
Now the real way to win is get a build order. Choose a race that fits your style, I'm sure you already have an idea about this. Now here's the tricky part, a build order is essential to your strategy but don't let it lock you in. You must scout at all times to know what exactly your opponent is up to. There is no sense in continuing a strategy if he can counter it easily.
Once the initial battles are fought, if you scouted right and your strategy was sound, you should be keeping him pretty bottled up in his base or you may have won already. If he's still hanging in there, it's time to expand. If your initial build order didn't include taking your natural (the first set of crystals outside your main base, it's called a natural b/c that's the natural place to expand), do that quick and go look for another spot. If the map wasn't filled, an empty base should be perfect.
Resist the urge to turtle (build -stationary- defense, never attack), this will kill you. Build lots of units (make sure you get some stuff to hit air, just in case) and go hunt his expansions and don't forget to check up on his main to see if he's gearing up for an attack, good recon is priceless. If he isn't expanding aggressively, try to contain him. Nothing huge, just a big enough force so that he'd have to work at breaking out, which would deplete his forces and give you early warning as to a possible attack.
While you're pestering him with your current force, those new expansions should be making you a very rich man. Use your income to build more factories and upgrade your attacks. Don't try to make a big army of a single unit. Spread out to include artillery type units (I prefer templar but reavers are good too) and air (avoid scouts btw). Make sure to use your recon to tailor your force to his base. If he has lots of bunkers tanks or whatever, bypass it with shuttles. You can clean out his stationary stuff later, destroy his infrastructure and you'll have won the war.
Fight some 3v3's, spectate some games, get some experience. You'll learn how to win but it will take time. Oh yeah, avoid BGH maps, it destroys the "expand or die" philosophy that is present in every other SC map. You'll pick up bad habits and your skills will only suffer from it.