Air Power vs. Ground Power
Learn to understand the role of typical flying units. The basic flying unit for each species, the Terran Wraith, Zerg Mutalisk, and Protoss Scout, is useful for precisely two reasons.
First, it can fly, and therefore may travel anywhere on the battlefield uninhibited. Second, it can safely attack any ground units incapable of firing back. These include close attack units like Protoss Zealots and Zerg Ultralisks and artillery units like Terran Siege Tanks and Protoss Reavers.
Flying units are very expensive. Unless you have specific use for these two features, do not waste your precious mineral and Vespene Gas reserves on fliers when you could be spending them on ground forces. Certain types of battlefields, either consisting of isolated islands or of winding canyons, cater to the commander who maintains air superiority. But ground units, who tend to deal much more damage and cost much less than flying units, will dominate wide-open maps that allow uninhibited travel. The rule to follow is: do not build flying units unless you are certain they will come in handy. And only under extremely rare circumstances should you consider building flying units prior to first establishing your ground forces.
The Lifeline - Your Gatherer Units
Ironically, the most important unit at your disposal just happens to be the unassuming gatherer. Gatherer units - the Terran SCV, Zerg Drone, and Protoss Probe - are responsible for constructing new facilities and collecting resources. Building plenty of gatherers and maintaining a consistently fast flow of resources is vital to your success, as you must continuously build new units and research new technologies to survive. Often, he who gathers the most resources during a given battle ends up winning. At first, you want to maintain a ratio of close to two gatherers per one mineral patch and have enough gatherers working a Vespene Gas geyser so that a gatherer resides within your Refinery-equivalent at any given moment. Try and match these ratios for your first expansion base, but taper off a bit for your following expansion because by the time it is prepared, your gatherers at your original base will likely be running short on resources, and you can redirect them to your new site.
You must do everything in your power to keep your gatherers safe. If your enemy attacks and destroys them, your economy grinds to a halt. Not only that, but replacing large numbers of gatherers is an expensive and time-consuming process. In turn, you must use every means at your disposal to strike the enemy where it hurts him most - his own supply line. Killing enemy gatherers will make his economy slow severely, and even as you continue to generate resources, he will only lose them as he tries to replace what was lost.
Gatherers are weak and easily killed. Powerful area effect attacks like the Protoss High Templar's Psionic Storm can kill or injure many gatherers at a time. Artillery units like Terran Siege Tanks and Protoss Reavers can also kill many gatherers quickly. Should you manage to sneak such units behind enemy lines, it will be more than worth your while to sacrifice them in destroying scores of enemy gatherers. Protect your own gatherers by placing a few defensive structures near their supply line, but most importantly, be aware of the immediate area and keep an eye out for incoming enemy transports looking to slip into your base and attack you where you are vulnerable. If the worst should come to pass, order your gatherers to flee the area - or if your enemy force is relatively weak and no other reinforcements are nearby, you may order your gatherers to attack. They can kill small numbers of low-level units like Zerg Zerglings and Terran Marines without taking too many losses.
Next: The Ultimate Power - Your Special Ability Units 
