Well, part of the equation is what kind of games you like to play.
If you are a very casual player, then it is best to find a like-minded clan with whom you can enjoy games. Pick-up games (even multiplayer casual formats) are often cut-throat (or at least one player in the game will be).
I would recommend starting at
Pure MTGO, they have articles on everything MTGO (and are MTGO exclusive, so even the format articles are from an MTGO perspective).
There's an ongoing conversation in the EDH community
here about why players do or do not play on MTGO. The whole thread is interesting, but in particular,
this post has some good BOT information.
If you see me on and want to meet up, I can gift you a few hundred cards to help kick-start your collection. I'll add more tonight if I think of anything else.