After logging in on terminal with user name and password, how do I start the GUI?

Ubuntu Server does not include a graphical user interface by default. If you are connected to the system console you will see a screen similar to the following:

To login to Ubuntu Server from the above screen:

1. At the login: prompt type your username and press Enter.

2. At the Password: prompt type your user password and press Enter.

To install the default Ubuntu desktop environment (Unity) in Ubuntu Server run the following commands:

sudo apt-get install ubuntu-desktop
sudo reboot

These commands will also install the LightDM greeter so that you can login to the Ubuntu desktop environment from a nice login screen.

If you logged in at the system console, one of the following two commands will bring up a GUI login screen to login to the Ubuntu desktop environment.

sudo systemctl start graphical.target # Ubuntu 15.04 and later

or

sudo service lightdm start # Ubuntu 12.04/14.04