Cellbomber said:
I tried Zerg but had no clue what I was doing. I should just stick with ter at first since I'm familiar with them from the campaign.
Play against easy AI to get a feel for all the races. Really it's the only way to get Multiplayer. First learn the basics of all 3 races, how they work, what they can do and what they can't.
You don't have to know all the stats, but for example you should know that a viking has more range than a mutalisk or corruptor, and equal range to a carrier before he launched his interceptors.
Once you played all three races and understood how they work, pick one and stick with it. Play against harder AI and finally against real players. You can look up various build orders on the Internet (liquipedia) but at first, the most important thing to know for multiplayer is:
Build Drones/Probes/SCVs constantly,
build Pylons/Overlords/Supply Depots constantly,
keep you money low at all times (below 500 if possible)
You will quickly learn how many barracks you have to build that you can support with your income from one base (hint: it's 3 when you have 1 reactor and 2 tech labs for marauders, it's 6 without addons and constant marine pumping)
Just play and you'll find out.
Once these basics work, you can try to do funky stuff
Also always ask for advice if you need help. People will gladly help you.