Okay, I suck at the game but something that is helping me is completing the single player portion of a game I've owned for a decade.
The first starcraft. The second game is a faster paced different sort of beast but without any real way to learn the ins and outs of units you pretty well , playing through the first game is your best bet. I too felt the zerg to be to overwhelmingly different from Terran to really get a feel for how to properly utilize their forces.
I'm still not quite done with the game obviously (I'm on zerg mission 8 at the moment) but what I have learned in SC1 transfers into SC2 quite readily, the basic units are still mostly the same across both games or at least in SC2 there is something that's a replacement.
At the rate I'm going I'm no longer sure I'll have the SC1 campaign finished in time to really dig into zerg/'toss more in the SC2 beta but yeah, learn the basics in the first game, I'd say finishing up to mission 4 bare minimum.
