Don't do anything. Just wait.
No, really, I'm not kidding. For the beta test, there'll be some note up on this site (and most every flight sim news site will mention it I'm sure). My guess is that you will download the software (probably around 4 Meg), then run it. There will most likely be a built in "create account" option. Now, since the beta is free, I'm guessing that they'd just like to keep tabs on who is playing, and make sure people aren't creating multiple accounts or something...
Anyway, just sit tight and the beta should be up for download in not too long...
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