If you’re from the US, you can sign-up here. For EU users, sign-up here.
Of course, do remember that signing-up won’t automatically enter you into the beta, it’s just that your name will be in the hat once they make their picks. In saying this, it wouldn’t surprise me if everyone actually got in, though.
Do remember that this is a beta and NOT a demo. There will be bugs, the game will crash and understandably, some (or most) of the stuff in the beta are just placeholders for the game’s final version. Oh, and make sure you report all the bugs, glitches you encounter to Sony! That’s what’s a beta is for, after all.