The ability to gather and effectively use information about the enemy is of the utmost importance. In strategy games, this is known as scouting. Practice it until it becomes second nature, for it will allow you to know where your opponent is, what is he doing, and how to respond to his tactics. If you know your enemy, you will be able to locate and destroy expansions, meet his armies with counter-forces, and even predict his next course of action. Through proper scouting, victory will be assured.
In 1 vs 1 matches, most players send out a worker to scout the map very early in the game. In team matches it is not uncommon to have one person send a scout while the rest of the team focuses on construction and unit production. Sometimes, however, every player sends out a worker to a different location in order to cover the map quickly. The sooner you find the enemy, the sooner you will be in the position to decide what the best course of action is.
Keeping Tabs on Enemy Offensive Building
Use your first scout (remember, scout early, scout often) to check for enemy workers or structures in the vicinity of your base. Be especially wary of protoss pylons and terran bunkers; don't let them set up shop.
Watching the Enemy
Keep tabs on the enemy's main base to see what units and structures they have. Also, watch those resource spots. You don't want the enemy expanding without your knowledge.
Stopping many strategies is often as simple as catching the enemy while attempting them. Keep an eye out for anything out of the ordinary.
Last but not least, familiarize yourself with the tech trees of all three races, even if you will only specialize in playing one of them. This way you'll know what to expect and how to properly counter it.
Starving the Enemy, Denial of Expansions
The key to victory lies in securing resources for yourself and denying them to your enemies. More resources translates into more, and more expensive, units and upgrades. If you can prevent your enemy from expanding, he will not have the ability to sustain his production and you will eventually overpower him through effective unit usage.
Many inexperienced players don't realize this and wonder why they lost. Usually, it is because their opponent mined more resources than they did. Have a look at the game stats, check the replay, find out what went south, and then get back into the fight.
Using Structures as Scouts
You can use structures to keep watch over certain areas, or use flying structures to explore the map.
The purple terran player built a supply depot to be able to watch the area.
Here are the most common and effective scouts for each race.
- Workers (scv, drone, probe)
- Scanner Sweep (ability at the orbital command center)
- Flying structures (barracks, factory, starport)
- Zerglings (great cheap scouts)
- Changeling (overseer's 'Spawn Changeling' ability)
- Creep tumor (spreading creep gives more vision of the map)
- Burrowed units