Step 1: Log on to your Battle.net account. If you don't trust our link -- and you should always be careful about clicking links that ask for your password -- just type us.battle.net into your browser bar and click the log in option along the top of the screen.
Step 2: Head to the beta management page. If you used the link above, you're already there! Otherwise, once you log in, you'll have to click Account in the top bar and Beta Profile Settings on the right-hand side of the screen.
: Update your system specifications. If you haven't done this before, you'll be asked to create a beta profile by giving Blizzard some information about your computer. If you have done this before, but have upgraded your computer since, you'll probably want to update your information.
To do this, you're asked to download an application (for Mac or Windows) that checks out your computer and populates your profile. Once you've downloaded the application, unzip it, run it, and follow the (simple) on-screen instructions.
: Select any beta tests you'd like to be considered for. Once the application has run, it should open a Battle.net page showing your system information and allowing you to select any beta tests you'd like to be considered for. Make your choices and select Update Preferences to save.
While this doesn't guarantee you'll get in when the beta starts, signing up is your only way to have a chance. So good luck!