If youre sincerely interested in getting into a fighting game's online community you've got two things that really help:
1) stick with one character, stick with one game. Whenever a new SF game comes out I pretty much stick to Ryu until I've seen enough of the other players online to see how they're used and how to compete against them. In this example, it helps that a lot of Ryu's fundamentals carry over from game to game. If you were good with the character in SFII/SFII/SFIV odds are good that you'll be decent with him in a new fighting game, Scorpion/Subzero are similar franchise characters in the MK series who don't see dramatic shifts in their styles from game to game.
2) buy the game day one and get into the online scene immediately. Since almost everyone is a newb on day one, expert and newbie alike, it's a lot easier to learn the game when the playing field is the most even it'll ever been than picking up the game months later.
Fighting games tend to be the exception for me in regards to day one purchases because of the second reason. Just letting a month pass means a lot in so far as the meta game is concerned.