Online games are natorious for being the most complicated games to make, mostly because you programming framework has to be completely different. You need to make essentially 2 different programs. One has to be server side and it deals with the packets sent from the client, it does all the calculations, keeps tabs on player id’s etc. then periodically (the time being determined by the type of game) it has to update all of the players.
The second program you have to make is the client side, which receives updates to show the player what the current game looks like, this part usually goes through 3 steps.
keep in mind this all sounds a lot easier it is. Compound that with the fact that you need to send all you information back and forth as efficiently as possible, you really have a hard task ahead of you if you go through with this. I’d say make a few games. Once your to the point were you have
A. released a game or
B. worked on a released, successful game.
once you have done that, read up as much as possible on game networking hierachies and at that point you should be ready to start working on an online game.
PS. once you get to that point, don;t start trying with an MMO, try some small online game.