My advice- play the campaign first
It helps you get a gradual experience playing the game, instead of diving right into the middle of a ladder game where it would confuse the ____ out of new players.
It also lets you enjoy the awesome storyline of starcraft ^^
After campaign, play a couple of matches, watch some Day9, get some friends to help you with your strategies and try your best to improve what you're doing poorly. Practice practice practice.
(note: not all units in campaign are in multiplayer)
My info in my sig if you need anything ^^
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