To register with the site click on the "register" link at the top right of this page.
Username & Display Name
You will be asked to provide both a "username" and a "display name". The username is the name you will use to log in. The display name should be the one that is shown everywhere on the site. So when you post to a forum it will be your display name that is shown. but it seems not to be the case! It may be sensible to pick a username the same as the display name, for example forename_surname: john_smith.
The registration process needs to verify your email address and so once you have entered your user details and chosen a password it will send you an email. This email contains a verification code and a link that you must click on to complete registration.
This might seem like extra hassle but it ensures that all users have a valid email address and hopefully encourages bona fide people to sign up rather than spammers who want to abuse the site.