1) Scout your opponent well. Send out that worker around 10 supply count. The point of the early scout is to find him if it is one of those maps and to make sure they are going for the standard build. Some things to look for are an early gas. This indicates that they will probably be teching up.
2) Know the builds of all races. You need to understand what your opponent is going for. Know the common strategies so you can piece together the information you get when scouting them. If they are Terran and you scout two early Refineries then that can mean early cloaked Banshees so be prepared for something like that. You can read about all of the race strategies and what buildings mean what.
3) Know the units of all the races. Each combat unit in the game does well against some units but are weak against others. For example Terran Hellions do well against Zerglings but do poorly against Protoss Immortals while Zerglings do well against Immortals. This is what the rock-paper-scissors of Starcraft 2 is all about.
4) Use hot keys. To become a faster player and be able to multi-task better you really should learn about hot keys. It will make you faster. At first it will be difficult but after using them for about 10 games straight they should become second nature. Learn about setting control groups and what hot keys pertain to your race.
5) Know what play style you should be doing. If your opponent is expanding early while you are massing up units on one base you should know that they will be slightly behind in their army compared to you since they will be pouring their resources into their expansion instead of an army like you did. That means playing aggressive and attacking them will be a great way to win.
Be sure to practice, practice, and practice some more. Whenever you start something new that you are not used to at all you will not be very good at it. It is just the way it is. But overtime with some dedication you will get good.
What Starcraft 2 is also about is bluffing your opponent. Let's just say that you are Protoss facing a Terran player. Each of you obviously wants to find out what the other player is going for. That way you know the best counter attack.
You need to be able to prevent them from scouting any buildings or units so they have a difficult time getting an accurate read. Not only that but you should know when a good time to attack would be. If your opponent is just expanding and is about to saturate his second mineral line while you are building up a strong army them the best to attack is sooner rather than later.
If he expanded before you then he can get ahead in economy if he has that expansion for a long enough time to build a strong army to counter yours. One base aggressive play will usually beat a player going for an early expansion. While his resources are going towards the new base he will likely not have enough resources to get enough defense to fight off your army.
But you need to attack at the right time or it will be too late. Same thing for when you have to expand. At some point you will have to get a second base or else your first base will be all mined out. You need to defend the natural expansion before getting one down their.
Good defensive play can be done with proper placement of your army as well as Photon Cannons, Bunkers, and Spine Crawlers.
Know your opponent. If they are getting early air units you will need anti air units. If they are teching up then you should either be rushing or get advanced units to counter their units. If they are playing defense then expand early.
Not only should you have all of this in mind but you should be throwing some curve balls while countering any that come your way. Harass their workers with Hellion, Dark Templars, or burrowed Roaches. This is not cheating. This is smart plays that will damage their economy giving you the advantage.
Getting better at a video game like Starcraft 2 takes dedication. It is not something that just comes to you like that. If you watch the pros play at tournaments then you should understand that to get that good they did not just practice for an hour a day.
It is something they truly love doing. So they practice for hours and hours. It takes up a lot of time. Of course you do not have to play that much to get good unless of course you want to play at that level.
This game is all about skill and speed. Read about the game. Know the unit counters. Become a smooth player when it comes to controlling your army.
Proper micro is the key to becoming a better player. You need to be good at deciding when the right time is to retreat your army or to engage in battle.