General Tactics

The first thing you need to have in a game is an army, but in order to have a proper army you must have sufficient income of minerals and vespene gas. In a regular game you should have at least 15 (fifteen) SCV's/Drones on minerals. Depending on what your building you will need 1 to 9 SCV's/Drones getting vespene gas (on average). As you need more minerals and vespene gas don’t feel that you should save your population for you army. If you do this you won't have an army in the first place due to lack of funding. Build the extra man so you can have the better funding.
Now that you've got a good supply of minerals and vespene gas you should build either defense or military buildings. Depending on what kind of a player you are you will chose between the two.
To build proper defense you should have several things. First you need the main bulk of your defense. This should contain sunken colonies, photon cannons, or bunkers and siege tanks. The next thing you need (unless you build photon cannons) is to build detectors, whether this is in the form of missile turrets, spore colonies, overlords, observers, or science vessels.
One thing I forgot to mention earlier is that you should try to leave an opening in your defense. I don’t mean leave an obviously weak gap either. I’m talking about an entrance so your troops can leave to attack they enemy. The only time you don’t need an entrance is if you’re going all air.
After you’ve built your defense or if you don’t build defense, you should start you attack army. If your seriously trying to consrtuct a massive army you'll need at least 8 of the militaty building your going to use. This allows you to mass produce men even if dont need to. Doing this gives you the ablility to build masses of men if your army is destroyed or crippled.
Most people will build one unit type and nothing else. I’m not saying that there’s anything wrong with this method but the enemy will often build units to counter yours. This leaves you with a disadvantage, what I like to do is build a lot of men for the same type of guys. After this you attack the enemy with all of them, he’ll build guys to counter that unit. While he’s doing this you’re building a totally different unit. By changing units often he won’t know what to expect, he’ll have to build against everything, which puts him at the disadvantage.
Another thing you should try to do as you become more experienced is to expand. This is a good tactic for many reasons, one it allows you more room to build. This also presents a challenge, as you also have to defend your new settlement. On the good side killing you has now become twice as hard. Giving you a major advantage. You now also have a backup mineral supply incase your first one was to die off.