If you're not a PlayStation Plus member, you'll have to wait your turn. A full public beta will open up on February 21st.
The followup to 'Warhawk,' which has simply had an "early 2012" release date in the past has finally been nailed down to May 8th. Not exactly early, but at least it's a firm date.
If you're a PlayStation Plus member, or if you participated in the closed beta, you can get in on the public beta now. It's free to join and you'll be influential in finding bugs, balancing the game, figuring out server traffic and more. And, of course, you get to play the game.
Not a Plus member? If you got a 'Starhawk' beta key with 'Uncharted 3' you'll get in on January 31st. If not, there will be giveaways and other ways to get in up until February 21st when the beta goes live for all PSN members.
Source: PlayStation Blog