Games and NPCS for credits, quests etc.
Click the corner "help" button when you're on VMK for some quick help, and the FAQS on the homepage for more.
And for the what do you do in it question, well, you can do anything. Why would you join a game if you didn't know what you'd do in it! You play! Make your own rooms, create quests, explore the rooms, chat with new friend etc.
There are stickies on top of the VMK boards that have lots of great info on what to do also. Especially check out the guide Maleficent2 put together on the Virtual Magic Kingdom (VMK) Game Board Welcome to VMK !
milkabum, I joined the game in November 2005 even though I had no clue what it really was, what I was supposed to do, or how I was supposed to do anything. So plenty of people join without knowing how it all works. I'm living proof.
topaz, I'd suggest just walking around the Kingdom, trying out the mini-games, etc. See what you like and what you don't. Players Guide here is helpful, as are DISers. You may find you really like the mini-games (Pirates, Haunted Mansion, Fireworks, Jungle Cruise), or you may just like visiting the various areas or building your own guest room. It is whatever you make it. Hope you have fun!