What packages do i need to install on an ubuntu desktop for it to "become a server"?
You can install such server packages on a desktop system using the apt-get tasksel mode (don't forget the final ^):
-
LAMP server (Linux, Apache2, MySQL, PHP)
sudo apt-get install lamp-server^ -
Mail server
sudo apt-get install mail-server^ -
OpenSSH server
sudo apt-get install openssh-server^
Note that you can check which packages are going to be installed using apt-get install --dry-run <name of the task>^ before calling the above commands.