First, if you aren't in the Battlefield 3 Open Beta go to http://store.origin.com right now and download the Origin software. If you're used to Steam this is the same but EA is now using it's own proprietary system for game distribution.
After you create your account click on 'Store' at the top of the screen and click on Battlefield 3 Beta. Let it download, the system is pretty quick actually (we were getting 900k-1mb per second).