Instructions
Play the game often, making sure to gain play exposure with all three playable races. Even if you are not focusing on a specific race, it is important to understand that race's main game mechanics. Doing this will provide you with a much better tactical understanding of how to approach that race when playing against it.
Join an online "Starcraft" community and arrange games against more skilled opponents. Doing this can provide you with great teachable moments, particularly if a veteran player is willing to evaluate your play style and give personalized tips and tricks.
Watch game replays of matches you won and lost to see what you did right or wrong and to examine your opponents' strategy. Game replays can be watched by creating a custom game then clicking the replay button and choosing the correct replay file.
Investigate the Internet for strategies and tips posted by other veteran players. There is a treasure trove of information about "Starcraft" strategy on the web.
Be patient. Just starting in a game that has been out for more than a decade means you are going to play against players whose understanding of the game and its mechanics dwarfs your own. Only through perseverance and patience can you manage to break through the "newb" bubble and hold your own.