When registration is enabled for an event, you can register online from the event's public page. As long as there are seats available, simply need to fill out and submit the event's registration form. This will add you to the attendee list and send you a confirmation email.
If an event is full, however, you will instead see a message saying that registrations are fully booked.
If there is a waiting list for your event, you will have an opportunity to sign up for that instead. If one of the attendees cancels their registration, then the next person at the top of the waiting list will automatically be registered for the event.
How to cancel a registration
After registering for an event or signing up for the waiting list, you will receive a confirmation email. That email will contain a link that will allow you to cancel your registration online.