Ahh, Starcraft. The most funnest game I ever have played. I'm talking about the first one, if course. Now, since I'm lazy, I won't explain how to play the game, but instead will teach you how to properly use each faction. I'm basing this off of Brood War, mind you.

Terran

Against Zerg
-Too commonly do people overlook Firebats. They are very effective against Zerglings, Hydralisks, and pretty much any land-based Zerg unit.
-Take out Overlords as fast as you can! It takes away 8 units from the Zerg player, and makes way for cloaked Wraiths, and Ghosts.
-Guardians got you down? Use a defensive perimeter of Wraiths/Battlecruisers!
-Missile Turrets are probably he best things you can use against Mutalisks. Keep them in groups of three.
-When attacking: use 12 Firebats, and 12 Goliaths. Firebats take care of the ground units, and the Goliaths clear the skies.
-Drop off a Ghost behind the enemy's mineral field and target a nuke where most of the drones are, and if you can, right next to the hatchery. It will pause Zerg production for a while, opening up a chance for an attack

Against Protoss
-But on the flip side, only use Firebats for a Zealot Rush. You'll need them.
-Upgraded Goliaths can effectively take out Scouts, and in bulk, even a Carrier!
-Bunkers of four Marines each can easily take down Dragoons.
-Put Missile Turrets at the perimeter of your base! If it ven prevents one Shuttle from dropping off a pair or two of Archons, it was well worth it!
-Never leave any of your SCV's outside your base. If a Dark Archon comes along a Mind Controls it, you just might be fighting against the Terrans AND Protoss!
-A well placed Science Vessel's EMP burst can drain a whole Protoss army's shields. Goodbye Archons!
-Use Battlecruisers for picking off Zealots. All they can do is run around before they're shot off.

General Strategies
-Siege Tanks and Bunkers are an epic combo.
-Sensor Sweep an area you're going to start a base in. They're might be Observers or Overlords there, or even worse, Lurkers.
-Mobile structures means a mobile base. It's possible to defeat a whole base by moving a Factory to a ledge above the base. I've done it.
-Don't be skimpy when producing Marines. They are your bread-and-butter unit. A player without Marines is a player who is probably losing.
-I don't care that they're expensive, Yamato Cannon Battlecruisers own all! Except against Scrouges, that is.

Who should I do next? The Zerg or Protoss? You decide! By the way, if you haven't played Starcraft yet, play it NOW. You'll love it, trust me.