MySQL should have been installed when you selected the packages. Now you just need to start it and make it start automatically everytime the computer starts. To do so:
Now check that networking is enabled. Run:
You should see a line like this:
If it does not, edit /etc/my.cnf and comment out the option skip-networking:
#skip-networking
Now, assing a password for the root user in MySQL, and replace the user root with the user mysql. I'm using the password mythtv.