Howdy. Thought I would toss ya a few basic tips...
1. Decide what you feel like doing. Want a huge ground force? Then plan on a bunch of Gateways/Webgates. Feeling void rays and carriers? Build enough of your stargates to crank those things out fast.
2. Usually, with the Protoss and Terran, you will want to Wall off the Front of your base. Barracks, gateways, forges, Supply depots, all that stuff. Can really keep annoying things out of your base.
3. Everyone has said so already, but it bears repeating. Be constantly spending money. If you find yourself trying to make more than 2 guys at each gateway or stargate, build another one, cause you are not pumping guys fast enough.
4. Scouting Worker. Find out what they are trying to do. Looks like Roaches? Then you will want immortals/Marauders/Roaches of your own. Look like an air build, or quick expansion? Perhaps you should rush, or prepare air defenses.
Here, thought that might help you out.