Instructions
Launch the game and sign in to play the online practice matches. Click "Multiplayer" at the top and then click the yellow "Find Match" button. While playing against the computer will get your feet wet, it doesn't prepare you at all for facing a human opponent. All new "StarCraft 2" players have the opportunity to play 50 practice matches before joining ranked matches. Practice matches have no effect on your future ranking.
Alternate between each race every game, until you have 30 matches left. Playing each race lets you become a well-rounded player and also provides you with the strengths and weaknesses of each race, which you can exploit later. Once you have about 30 matches left, you should have a feel for which race you prefer. Focus on playing your preferred race for the majority of the remaining matches.
Look at the replays of each game you lose. "StarCraft 2" automatically saves a replay of each game you play. You can access the replays by clicking "Replays" at the top left of the screen. Replays give you a view of both you and your opponent, which gives you the chance to dissect where you went wrong.