Check out this great article to get started using the RPi nano text editor via raspberrypi-spy.co.uk:
If I need to edit text files directly on my Raspberry Pi my text editor of choice is nano. There are other text editors available but I prefer nano’s relatively straightforward interface.
As a command line based utility it may feel strange for users who are more familiar with a graphical interface but it is easy to learn the basics. Syntax colouring is available which makes reading and reviewing scripts easy.