Guild Wars 2 open beta sign-ups are open for only 48 hours, so if you want to get in, you better hurry! It does not guarantee you a spot, but you'll be notified of any future beta events. All you need to do is to submit your system information, which they do for you, and your contact information by 6:00p.m. (UK time) on 24th February.
ArenaNet has also posted an extremely detailed FAQ about the open beta. You can sign up for the open beta here. Go quick, you have less than 48 hours!
Image source: http://nerdtrek.com/pax-2011-guild-wars-2-panel/