A Quick Start
For a slightly quicker start, you can use this method: first, once your screen turns dark green/black (after you go through the loading process), select one of your builders by pressing the builder hotkey. Then, move your builder to the top of your screen and, using the hotkey, build a building (you typically start with a resource building, but that's up to you). Repeat the process on the bottom of the screen with your other building.
This won't make or break your game, but it can give you a two to three second advantage over your opponent, which can be very helpful!
B = Builder
R = Resource building (not for all races, be sure to check your manual)
C = Basic barracks (once again, not for all races)