You just have to visit minecraft.dk and then follow the link to the forums. There's a section dedicated to applying to join. Follow the guidelines and put in your application :). It's that easy.
After that you'll be on trial for 48 hours, go ahead and find a plot (don't be afraid to ask for help), and start building a house. Once you've got yourself a decent set up and the 48 hour trial is over you can go through the process of being approved and boom you're in.