Many beginners make the mistake of trying to use every race when you should only use one. When you pick your race whether it is the Terran, Protoss, or Zerg you will have to stick with them for a long time until you dominate them. Getting in the game with your favorite race is usually the way to approach this game Starcraft 2.
This game is much easier to get good at when you master just one of the races to start off with. The race you choose will be yours from here on out. Millions of other Starcraft 2 gamers will be learning your race and try to dominate battle.net with them. Get a Starcraft 2 guide for more insight. Play with your race until you have reached a good ranking online. You can easily master the race by first getting down their basic skills. Make sure you understand the ins and outs of every unit so building good armies will be no problem. It makes perfect sense to become a master at knowing what each race and unti is capable of.
Know what every race is capable of to be real good at this game. The time will come for you to attack your opponent so be sure to throw everything you got at them when the time comes. Basically the more you learn about the game strategy as a whole the more effective practices will be. If you want to be diamond then there is no other way around having to master the basics.