I prefer MacPorts for installing MySQL, since I get always a recent version, while it's installed in a safe and accessible location.
If you installed it through MacPorts, there's a script that creates the initial databases and passwords (if I remember well):
/opt/local/lib/mysql55/bin/mysql_install_db
For starting the server, I simply do a:
sudo /opt/local/lib/mysql55/bin/mysqld_safe &
Of course, I use it just for developing and testing purposes, so you'd probably want a more specific configuration when exposing it to the internet.