Now that Starcraft 2 has been out a while and non-beta players have managed to develop some tactics, or just steal them from long time players, I'm going to share the easiest tactic for Terrans.
The Terran Marine Rush Tactic.
Marines are the first purely military unit you will create and the easiest. All you need in order to recruit a marine is a barracks.
Now, you might think that marines are the weakest unit so they are not worth making, but with this tactic the secret is in the speed. Marines are quick and simple to make and you can make dozens in no time at all.
Start off creating SCV's, until you have 10 and then build your first barracks. Then build a supply depot, then a barracks and then a supply depot and then a barracks. Make sure you have upgraded your command centre to harvest more minerals. You can repeat as often as you see fit, but there is one thing you must know about rush tactics...
They can be seen as cheese.
By cheese I mean, they are an all or nothing tactic. Many Starcraft 2 players hate these tactics and will abuse you for using them, especially the Terran Marine Rush. If you send 20 marines to a new enemy base and he or she manages to fight you off, you may be in a whole heap of trouble as you will be behind technologically.
The good thing is, rush tactics take no time at all. I've won games with this tactic within 7 minutes.
After three barracks have been built, as long as you've been clever and making marines at the same time you should have in the region of 20 units ready to go. Set an SCV off making another command centre and send your units in.
By the time your units have got to the enemy you've probably been able to replace their numbers from your growing fleet of barracks.
This being pretty early in the game your opponent will probably still be working on pushing his tech trees and building lots of advanced buildings, which you'll shoot to hell and back.
Rinse and repeat if necessary. Often, laughably, you'll find that your opponent has sent half a dozen units to your base in a pre-emptive strike which your newly created marines will crush easily and then you'll be blowing his undefended base to smithereens.
More advanced tactics, including the Starport Rush are available in Starcraft 2 Secrets.
