Before being able to join a tournament, you need to create an account. Unlike the tournament software you might know from in-person tournaments, where tournament organizers input results and print out pairings, everything on here will be online and personalized. Players are able to register for tournaments, submit decklists and report games on their own, for which user accounts are essential.
The optional fields in the signup form can be filled out if you want to enter tournaments with your real name, but you may also ignore them and add them later in your account settings if needed.
If you run into any issues while creating your account, please take a look at the Accounts FAQ.