User Account
Your user account can be what ever you want it to be. But if you use the same username that you have used on other places, then you are vulnerable to bad actors as they would be able to track you down based on that.
Email
To combat forum spammers, email activation is required on this forum. You can use a temporare email if you wish, but as emails can be used for password recovery it is always better to use an email that no unknown person can gain access to.
If some unauthorized person would get access to the database, then they would be able to learn more about you by reviewing the email you used for signing up to this site.
For that reason, the best thing you can do is to create a new and fresh email for this site.
If you contact us elsewhere, then we can easily activate your account manually if you don't wish to use an e-mail or if you have troubles receiving an activation e-mail.
We do periodically activate accounts that seem legitimate manually, so if you sign up without using a real email you might still gain access to the forum after a manual activation.
Passwords
PHPBB3 stores passwords in a salted and hashed format, so if there is a database leakage it is really hard (very often, impossible) to crack a password.
But even though its really hard, it is still possible, especially with modern technology. For that reason, it's important that you use a password that is fairly complex, because salted and hashed passwords need to be bruteforced or guessed (bruteforce attack / dictionary attack).
An other important aspect to think about is to have a unique password that you haven't used anywhere else on this site. Because there are a lot of websites and databases that have been hacked before, and if you use the same password on one of those, a con-artist could mix and match your nickname here with your nickname on other places and gain access to your account.
IP
Your IP address is stored in the database, that is a necessary feature for a forum to function properly, not only from an anti-spam perspective, but also from a forum administration perspective (for keeping bad actors away).