StarCraft 2 is a game of speed and reflexes. You've got to have a strategy and act on it as quickly as possible and faster than your opponent to have a chance at victory. Consider these 3 most important StarCraft 2 techniques to be as competitive as possible and always beat your opponent.
The first of these StarCraft 2 techniques is that it's essential that you focus on one race to play as. I know a lot of people who like to play as the different races to get the full rounded experience of the game, but when it comes to competition are typically not very good because their skills are too spread out. This is why it's typically recommended to just focus on one race and develop your skills in full with that race because each race comes with their own units, structures, and strategies for taking out their opponents depending on their make up.
Another of these StarCraft 2 techniques which is based off the first one is that it's important that you stick with one race because you can learn the build orders most effectively in doing so. The build order is the sequential order in which you build up your forces from the beginning of the game and depending on which strategy you go with, you might pursue different build orders even within the same race. It's essential that as soon as the game begins you start on your build order and commit to it just as your opponent is because if you fall behind at any point you've already given your opponent a significant advantage in that time which is difficult to make back up.
Lastly you should consult StarCraft 2 techniques guides to learn tricks used by elite gamers and early beta testers who personally learned the ins and outs which your opponent might not know about to give you a substantial advantage. Each race has their own set of unique StarCraft 2 techniques which you can use with their different units such as how each race has their own set of different rushes they can use. The store-bought guides typically lack in giving you useful information beyond the basics of how to do the campaign mode or set hotkeys. This is why user strategy guides written by actual players themselves are the best, plus they are typically updated with new strategies and info all the time rather than just the initial release.