Making an online multiplayer game isn`t easy AT ALL. Trust me. It`s like learning a new language. If you want to try though...
http://www.gotoandplay.it/_articles/multiplayerCentral/I wanted to make online multiplayer games once as well, but that time I sucked compared to what it takes to make something like this. You`ve got to know functions, arrays, objects and others in great detail! If you don`t know ONE of these, you have no chance right now. However...
http://www.fighunter.com/The Tutorials section of this site has tutorials that explain all of these in Flash AS2.
Basically, if you're a noob, just give up, get better, make some good games that aren't online, check each and every site, learn everything you can about Flash, and try again when you have more experience and you're successful. Check out the first link anyway, you might learn a few things.
If you are REEEEEEALLY good, then give it a try, and good luck!
_________________Previously buzzer465 - I just remembered this account existed after like 4 years...
Tall hat Kirby! Edited off the SSF2 Kirby.