2nd question first:
edit /etc/apache2/sites-enabled/default (it might be /etc/apache2/sites-enabled/000-default), and change DocumentRoot and Directory drectives. reload apache2
1st question second:
You can either use the file you just edited, and create a new one called your-domain.com (change to your actual domain) and place it /etc/apache2/sites-available/ or
(change your-domain.com to your actual domain/sub-domain you want to add)
I wrote this tool to make peoples lives easier. Don't just trust any code. Try this first: (change your-domain.com to your actual domain/sub-domain you want to add)
It will output what will be done. If you're comfortable with what it will do, just pipe it to bash (run the command as outlined originally)
I have vhosts document root set to /var/virtualwww/your-domain.com in my script.