Starcraft tactics... That's a HUGE topic.
Even for a single race, there are at least a half dozen strategies, and each of those can have an altered build order, depending on what you like, how you play, and what your opponent is doing.
I play Zerg, and while I'm no pro, I know a little of how to play.
If you want fantastic tactics, you're going to have to bite the bullet and watch some pros. There are many great sites that show them... I'd recommend watching youtube videos by Combat-Ex, who has over 100 first person training vids of all races, or some pro commentaries by Klazart.
As for my personal favourite strategies:
I like the 6pool... where you start the game, build your first 2 drones and just gather to 200. Build a Spawning pool at 6 supply, and then build one more drone. You'll be back at 6 supply and then wait. When your pool comes up, build all 3 lings. Next, build an overlord.
Your economy will be behind, but if you get a reasonable harass in, you'll even the score. The other option is that you can kill his scouting scv/probe and then fake the rush. He'll build a def, waste about 100 mins and you move to tech up.
I also like the basic strategy of a 12hatch/11pool. It's a really simple BO. You build drones to 8 supply, build 1 overlord, build another drone. You'll be at 9 supply and save for a bit. When your overlord comes up, build the 2 drones you can. Use one to scout. build your 12th drone, and then put up your new hatch at your expo. When your hatch is started, your 12th drone should come out and you build a spawning pool. You'll be sitting at 10 supply with 1 scout, expo building, pool building, and a full min line. Depending on the map, use your floating drone to build your extractor.
Finally, my other start is a fast expo. Build to 9 supply, build an extractor, start your 10th drone, stop your extractor, and keep mining. You'll be 1 supply more then you should have. Take your 8th drone to your expo and wait. At 300, build your expansion. You have to be VERY careful with this, as a push will kill you. You really should only do this if you see they're either slow, or building defensively... an example of a defensive build is if you're playing against a P player and they are putting a forge infront of their expansion. They'll have cannons about the same time your lings get there, so don't waste your time on lings. Get the expo, and try to get an economic lead.
There are so many more strategies for middle & end game... those are just starting strats and it's already a lengthy post. I'm sure there are better players who can improve my starts, but I can post more info later if there's any interest.