From Before I Play
- Starcraft multiplayer is pretty much all about build order. The good players have some set series of actions that they follow right at the beginning of the game, then they scout to figure out what their opponent is doing, then they have some build order to counter whatever they see occurring - rush if the opponent is turtling, counter-rush if he's rushing, expand if he's staying passive, that sort of thing. The best piece of general advice I can give you is to not be afraid to expand to secondary bases to increase the efficiency of your resource gathering and move some of your eggs out of their single basket. Also, scout early and scout often. You need to know where your enemy/enemies is/are and what he is/they are doing.
- The best thing you can do is play skirmish matches and focus on your early game, making sure that you can control all your construction units efficiently and get buildings and troops as quickly as possible. Once you're good at the starting portion of the game, you can start trying out your army combinations, figuring out what works against whom and how to micromanage your troops to overcome the enemy.
- You'll learn more against players, generally speaking, but will also be more frustrated, because many of the people still playing Starcraft at this point have been playing for years and are extremely good at it. Skirmishes against hard AIs can teach you plenty at the beginning, but getting good at fighting the AI won't make you good at fighting other players.