Why do I have no 'Shader' category when trying to add nodes? I am a complete beginner. So, I might be dumb - sorry!
I made a 3D model. Now, I want to work on its surface - which lead me to materials and shaders. So, I added a material and then opened the Node Editor. I pressed Shift+A to add a shader and I got the following:

There is no Shader in the list. So, could you please tell me how to get it?
The Blender version I am using is 2.79b
First, make sure you are using cycles.

On the info window 

select cycles as the render engine



Then go to the node editor.
Without any addons, the Node Editor can be used in three different modes:



You are looking at the compositor nodes.

Click on the materials button in the Node Editor header.  

Also make sure the Use nodes box is checked.