1. BUILD ECONOMY
Everything you build in StarCraft II requires crystals or vespene gas. Begin by having at least two Gatherers on each crystal deposit. As you grow, build Vespene Geysers and have at least three Gatherers on each.
2. RESOURCE GROWTH
Your economy is never finished. Keep building more Gatherers constantly, and once you've got your Vespene Geysers, expand to new resource patches. One tip for Terrans: build spare Command Centres within your existing base, and fly them out to new resources around the map, filled with ready Gatherers.
3. SPEND THRIFT
Spend resources the moment you get them and never queue units up. It's tempting to queue and make things easier on yourself, but you want all of your money going to work on creating or upgrading things right here and right now. If you ever have more than 200 crystal or gas, spend it. If you ever have more than 600 crystal or gas, you're an embarrassment.
4. ZERG RUSH
StarCraft's signature 'oh-shit' attack is simple to orchestrate. Build a Zerg hatchery that excretes larva, and then a spawning pool to turn those larva into Zerglings. Once that's up, spam the nasty critters until you've got a crew ready to scuttle. Rush now
5. STOP ZERG RUSHES
On the receiving end of a Zerg rush early on? If you're Terran, block the ramps up to your starting point with supply depots and throw out a barracks focused on churning out marines. They can hide behind the buildings and shoot the Zerglings. If you're a Protosser, a few Zealots can take down a rushing zergling force.
6. WATCH REPLAYS Every match you play is automatically recorded, right down to how your opponent moved his camera. When you're beaten, watch the replay, see the mistakes you made and where your foe went right. Copy them next time - it might not work, but it'll make you better.