Go here: https://beta.guildwars2.com/
You'll be asked to download a program called Scannertron (love the name) which is basically a small system diagnostic which will tell the folks a ArenaNet what hardware you're running & what version of OS and DX is on your machine.
Once you run it, it'll popup a window with your sys info and a button for you to continue the signup process. Be sure you give them a working email address! You'll only have to do this process once for any future beta tests as they will be keeping your application information on file. If you're chosen to participate, they'll email you with further instructions.
Good luck, everyone!
KKat
NOTE!!! Signups are only available for 48 HOURS!!!