The command you're looking for is
VBoxManage startvm "VirtualMachineName"
That does assume the software is installed and your virtual machines are in the correct directory.
To make it start on boot, you could do something like an init.d script. You can do that by copying one of the existing ones, editing it to your liking (you might also want to add a stop/shutdown function to turn the VM off before you turn the host machine off), and then register it with the OS using the command
update-rc.d VMControl defaults
This assumes your file is located at /etc/init.d/VMControl, and you have made it executable (chmod 775 ./VMControl).
Hope that helps.