What do I need before I can enter a tournament?

Three things, in this order:

• A Creta account, signed in on the site.
• Fortress W installed, and signed in at least once using that same Creta account. This is what creates your game account, and there is no way to register without one.
• A tournament whose registration window is open — the countdown on its page tells you.

Do the second one before the day registration closes. People regularly find a tournament they want, discover they have never launched the game with this account, and run out of time.