Mix Shader Fac doesn't work I'm just starting to learn Blender and watching this tutorial video that tells you how to use nodes and mix Diffuse and Glossy BSDF. I'm having problems with the "Fac" part of mixer, as it seems to update in "Preview" section on the right, but not in the actual editor :(

Also, in the video, the guy goes to Node Editor right away and everything's already there, but I didn't see any nodes in the Node Editor until I clicked on "use  nodes" in the materials section. Can that have something to do with it?

And now changing Shader color in the menu on the left doesn't do anything either, I have to go down to the Settings and change it in Viewport %\ ...

I'm so confused ...
Your current screen shot shows you are in Solid draw mode and that will not update to preview your material, change it to Material draw mode and you will get a better idea of what is going on. Even faster idea of what is going on is to set a border in the 3d view with Ctrl-B and to then press Shift-Z to change to Render preview mode inside the border, and that way you will see actual feedback as you change the shader.