You should play it in single player for about two weeks. Play all three factions and learn about their strengths and weaknesses relative to one another. Also, try to play some longer games so that you get to contemplate the full tech trees.
When you're ready to come online, count on getting steamrolled in your first few games. The online game is different from single player and much, much more challenging, but it's definitely worth it. The game does have a learning curve and just like any other online multiplayer game, you have to grit your teeth and take your nooby lumps before you become a competitive player.
You might start by looking for comp stomps--humans v. AI games. That way you could watch your teammates and perhaps get some friendly advice for them.