First of all update Ubuntu
# sudo apt-get update
The below command will install GNOME desktop
# sudo apt-get install ubuntu-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
If you want to install light weight desktop install xfce using the following command
# sudo apt-get install xubuntu-desktop
If you want to install KDE desktop use the following command
# sudo apt-get install kubuntu-desktop