How to install GUI desktop on a server?

Depending on which desktop you wish to install, you install the the meta-package that installs all the necessary packages. You can use apt-get or aptitude to do this.

sudo apt-get install ubuntu-desktop

installs the Unity desktop

sudo aptitude install kubuntu-desktop

would install the KDE desktop

Other desktop meta-packages are xubuntu-desktop, ubuntu-gnome-desktop, lubuntu-desktop, and edubuntu-desktop .

If you wan to install a graphical desktop manager without some of the desktop addons like Evolution and OpenOffice, but continue to use the server flavor kernel use the following command

sudo aptitude install --without-recommends ubuntu-desktop

sudo apt-get install ubuntu-desktop