Click on the Log in link here, or at the top right of the page. You will then be prompted to enter your username and password. If you haven't logged in before, you will need to enter your password twice.
You may also give your email address if you like. Other users will be able to send email to the address, but they won't be able to find out what the address is.
If you click the check box Remember me, you will not have to log in again when accessing the site from the same computer.