Testers are going to be selected on the usual three categories -- system specifications, age, and region. Other categories will be used as well, however they haven't been announced formally by Bioware. Those interested in testing the game will need SWTOR community accounts (forum accounts) before they can jump in over on the testing website.
Testing is also only open to people who are over 18 years of age, as Bioware is serious in finding those who will offer good bug reports to aid the development of the game. If you think you have what it takes, then cruise on over to the testing page and sign yourself up!
Editor's Note: The website is getting crushed right now (for obvious reasons) so if you can't get in, just try again later. There will be plenty of time for everyone to sign up!