Well technically every kid and teenager wants to do it but adults will tell you that you can't without money or a group. You have to know at least 1 or 2 programming languages and have your own server/site above that. Then you can use a mmorpg platform like eclipse (u can look up eclipse mmo engine on google) and when you are done connect it to your server. But using a platform will set limits where if you know a programming language (we're using c++ with the GDK[game development kit] and Dark Matter[resources and a help with creating online games] ) you can write your own platform and start your game development with that or write up the game as a program and upload to the server. AJAX or just Java is very popular for in-browser online games but with dark matter and the C++ GDK we're working around that. I'm not an expert at C++ though so its going to take a couple of months or until next year. Technically, because of all the false information and competition no one has a perfect or valid tutorial or something you can read up so the best thing to is look for a great 3d game engine/platform and construct the game. Then later you can look up away to make it an mmo.
Oh yeah, since we're using the GDK we get another option gba hacking doesn't give us- 3D GFX! Along with more realistic terrains (if we want) and much more scripting capabilities that will let us do virtually anything if we learn how to. You should start learning early so by the time you are 15, 16, or 17 you can finish your first mmo.
Seriously. Also, you really won't get much done without a group so always ask around!