You can sign in either by clicking to login with Facebook, Google or you can "register a new account" as seen below:
If you choose to login with facebook or google, you will be redirected and logged in. You simple then need to choose a username for your account.
If you are choosing to register a new account will you will see the screen below, simply enter the information requested.
Once you choose "register new account" you will receive an email confirmation, click on the link in the email and then you can choose a username for your account.
You're all set to start chatting! You will be automatically added to the General channel on the left.