I agree with Kerrick that online multiplayer is a very different game and that you're going to get beaten up when you first start out regardless of how much single player experience you have.
That having been said, I do think people should play the game in single player for about a month first. You want to be very familiar with and very comfortable about all of the mechanics of the game before you come online. You should also play a couple long games with all three races so that you're familiar with their strengths and weaknesses. Also, read the Strategy forum and flip through old threads. Also, consider watching some of the posted replays (though I think most are for Entrenchment) and study what the good players.
One more thing, make sure you understand what a neutral extractor is (an extractor in a gravity well where there isn't a planet that can be colonized) and how to capture them. Also, get used to starting out with one of the following three capital ships (as your first capital ship) depending on your race: TEC--Marza. Advent--Progenitor Mothership. Vasari--Evacuator Mothership (aka the Egg). Practice playing without building more than one or perhaps two captial ships as well. Also, when you start out, build enough military labs first (before building any civics labs), at the beginning, so that you can make your LRM ship (TEC needs 2, Advent needs 3, Vasari needs 1). Also, if you have Entrenchment, I suggest getting used to the "Fast Start" game option and play the game with all speeds set to fast.
Once you feel comfortable with the single player game then it might be time to venture online. Definitely learn the online game because it's so much fun and it can get boring playing against the AI, which really isn't very challenging. One major differencet that you'll have to get used to in order to be a team player is that in the team game, you try to do whatever you can to help your team even if you personally are being beaten. In single player you would probably just end the game and start a new one, but that's the wrong thing to do in the online multiplayer locked teams game. After all, if your team wins, you win too regardless of whether or not you personally were beaten.
