I'm no pro, but here's what I generally do early on.
Protoss:
Build probes. After the 9th one, put up a pylon before you build the 10th probe. Continue until you have 17 probes. You should be able to chrono boost three times to speed things up.
As soon as the 17th probe comes out, put up three gateways, followed by four pylons and assimilators. You should be back up to 300 minerals right around the same time the gateways finish warping in, so immediately start making zealots.
Keep unit production steady, and use additional minerals to start building things like a forge, a cybernetics core, and more probes for gas collection. Make sentries as soon as you have the gas for it, and research hallucination early on, as hallucinations can soak up a lot of damage before the other team has detectors, and a phoenix hallucination makes a great high speed scout. Their shields are also helpful, and their force fields can be good for keeping ground units in the main base while you destroy a nearby expansion.
In the original Starcraft, I was terrible with Protoss because I was so slow with them. In SC2, they're currently my fastest early-game race.
Terran:
Build SCVs. After the 9th (or maybe the 10th, I forget) SCV, put up a supply depot. As soon as he's finished with that one, have him build another one before he goes back to mineral collection.
Build 18 SCVs. When you have enough minerals (which should be before you're done building the 18 SCVs), put up two barracks. Then, while they're building, put up refineries. Keep up with SCV production until you have 16 on minerals, 6 on gas, and 2 out for building and repairing.
Meanwhile, as soon as the barracks finish building, have each make one or two marines while you send those two SCVs to the entrance of your base to put up a couple bunkers.
After you've made a pair or two of marines, add a reactor to one and a tech lab to the other. Then start pumping out a steady and constant force of marines and marauders.
Research the upgrades for marine health and marauder stun shells as soon as possible. Put up an engineering bay shortly after the bunkers, and keep up with those upgrades. Get a couple of factories with tech labs up as soon as you can manage, as siege tanks are good to have as soon as you can get them.
In both cases, expand early if possible. SC2 is all about expansions, more so than in the first game. On that same note, you don't necessarily have to worry too much about assaulting their main base. Leave observers near all possible expansions, or run occasional scanner sweeps on them, so you know when they're trying to expand. If you can stop them from expanding, you can just wait for them to run out of resources while you build your own forces and assault their main base at your leisure.