This is really easy :) Just go to the registration page.
You simply need to choose a username and a password, insert your email address and validate. You will then receive a validation email to activate your account.
Your username is your login to connect on the website, it must contain no spaces, symbols characters with accents. As it is unique, you must be the only one to use it. If another user already took it, you must choose another one.
You can decide later from your user settings page to complete your profile with your first and last name or either add a display name on your profile (see Manage my profile settings)