There is a red button in the header to the right which may say "Be a Propagandist!" or "Join!" or have just a red person icon, depending on your screen size:
Clicking on it will take you to the registration page.
- Provide an email address that will be used in communications,
- come up with a user name,
- add an avatar or picture,
- indicate whether you would like other users to be able to contact you (a process that is mediated through the site, meaning others will not know your email address),
- and then click on "Create new account"
An email will go out to the address you provided with a link in it that will help you set up your password, and that'll be it.