StarCraft 2 is about speed and intelligence. Keep these three StarCraft 2 winning strategies in mind to decimate your opposition.
First, you've got to choose one race out of the three to develop your skills with. In other words, it's in your best interest to excel with one race rather than being just alright with all three. Each of them has their own ins and outs and unique set of units and structures, so pick one to master and once you have you might consider trying games with the other two just so you can learn the methods of each of them so that you can better defend against them.
Once you've picked your race, you can focus on learning their various build orders. An effective knowledge of build orders are essential for StarCraft 2 winning strategies and this is essentially the order in which you build up your forces depending on which strategy you want to pursue in each game.
As soon as the game begins you should begin immediately developing your bases and forces, regardless of the strategy you are pursuing. The point is to pick one and stick to it otherwise if you're not developing your base as quickly as your opponent is, you leave yourself open to giving away the advantage of the game and you'll never recover likely.
This is a game of speed and whoever develops their forces the fastest is going to win, therefore again it is quite essential to have a working knowledge of how to quickly pick and follow up on a build order.
Also important is that you consult a StarCraft 2 strategy guide to learn the various individual tactics and strategies associated with each race. You can of course go with the official guide which was meant to accompany the game, but you're likely better off going with a user created guide to learn tactics outside of the campaign mode so you can learn how to better compete in multiplayer mode which is where most of the replay value of the game lies.