How to turn a desktop Linux box headless

You want to change the default runlevel, on ubuntu the way to do that is in the grub configuration.

Plagiarizing heavily from this askubuntu answer (go on, upvote it...):

For Ubuntu 11.10 and higher

Edit /etc/default/grub with your favorite editor,

sudo nano /etc/default/grub

Find out this line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

Change it to:

GRUB_CMDLINE_LINUX_DEFAULT="text"

Update Grub:

sudo update-grub