How to install desktop environment in Ubuntu

First you nee to make sure you have enabled Universe and multiverse repositories in /etc/apt/sources.list file once you have enable you need to use the following command to install GUI

sudo apt-get update

sudo apt-get install ubuntu-desktop

The above command will install GNOME 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 apt-get install –no-install-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