This might be obvious but if you want to kick ass as fast as possible, stick to playing one race. Also focus on learning to perform a basic opening (preferably not some stealth cheese or a turtle, but something relatively balanced between aggressive and safe) smoothly. Just spam that opening. When you're facing random people all the time, they don't know you have already done the same thing ten times in a row to others. When you develop enough consistency with the opening, you will have enough concentration left to scout properly, and won't fall easily to canned rushes. This ensures you reach into midgame often, get to practice your macro in addition to your micro, and when you start mixing things up, you have a familiar yardstick to compare to. You might have to go with 2-3 "main" openings because some of them will work quite poorly against specific races.
Take replays, watch replays. Initially your main interest will be "what was that rush I got rolled with, and how can I dodge it?". Moving into midgame, your focus will be "at which point and why did my opponent initially gain the resource advantage that ended up crushing me later?". Initially the reason is very often that the opponent expands, and you do nothing because you didn't scout him or connect the dots. You need to either expand, use your momentary troop advantage to make the opponent suffer bigger casualties in fighting than you do, or research tech that allows you to really drop the hammer on the opponent a wee bit later. In a pro game, the resource imbalances tend to be the result of successes and defeats in micro battles.
Occasionally, such as during a rush build, you'll be waiting for specific tech before you can build. (Say, storing 600 minerals, 600 gas and 6 unused larvae at your Hatcheries so once your Spire pops you'll have six Mutalisks in one Mutalisk hatch time.) These are exceptions. In general, troops sitting idle and lots of minerals (say, >400) unspent are signs of failure. Invest in mining more minerals, or invest in beating the snot out of your opponent.