1) Get a specific race. You may think that it's not hard to learn all three, but in practice, IT IS

2) You should have about 25 workers per expansion, and 2-3 unit producing buildings in your main for each expansion you have.

3) Try focusing on 1-2 different types of units and just mass those, along with 2-3 mobile detectors (ravens, obvservers, overseers) so you don't die to cloaked units.

4) Continued from 3, get weapon, armor, and research upgrades for the units you picked. So if you are terran and going marine + marauder, be sure to get the weapon and armor upgrades from the engineering bay ASAP, as well as all the ability research for them from the tech lab!

5) Use control groups. You do so by selecting a group of units or structures, then hitting ctrl-# (any number 1-0), then when you hit that number you automatically reselect that group. Probably the most important use of this is to group all your unit producing structures together, so even when you are controlling your army out in the field, one key brings up your entire production facility so you can quickly rebuild your army once you fight the enemy.

6) Oh and I think you don't understand zerg. The only building that zerg has to produce units is the hatchery, all other buildings are just to unlock new units and research.