Solution 1:

The easiest way is to install Ubuntu Server inside VirtualBox, it cames with no GUI by default, just command line.

If you change your mind and need a GUI, try Lubuntu,it uses LXDE a quite light GUI.

Solution 2:

If you have installed ubuntu on vbox than run ubuntu and press right-ctrl + F2

By doing this you will enter terminal mode and to come back anytime to GUI PRESS right-ctrl + F7

By doing this i think you dont even need to install new ubuntu you can run your current ubuntu without graphics and can practice

Hope this will help you

Solution 3:

You can try installing Xubuntu.