If you've been one of the lucky winners of a Beta Key at one of the many websites participating in contests and giveaways, you will have to discern between two options to access your account. But so you don't get lost in the dark without a leading light, we'll take your hand from here.
1. Account activation: In the eMail that you've gotten from the website participating in the contests should be explained that your account will be activated automatically. All you have to do is wait.
Apart from that you can download the client over one of the multiple sources here.