User Database

I've had an idea for a website project for a long time - basically a completely "from scratch" bbs. I have limited experience in building websites  so I'm asking for you expertize. What is the best system for building a user database for around 5000+ users? Also, how should I start learning to make such databases - the idea is I'd really like my own coded system. 

If you are really willing to help, point me in  the right direction to start learning the technique :).

ps. I was hoping the answer wont be mysql because it seems so complex but I guess all large systems are ;)

The performance arguments don't apply to MySQL and PostgreSQL any more. And even if they did, if you're pushing them that hard, you're doing something wrong.

However, MySQL is far more common on webhosts. That's the clincher.