How to save materials to image Blender

If by save the texture you mean bake the texture then make sure your render engine is set to Cycles.

UV unwrap your object so the texture maps onto it correctly when you bake it. I suggest that you do not have any overlapping islands.

Then you should be able to press bake with it set to diffuse.

Although, it seems that you have a diffuse shader set to a yellow color with it mixed with a glossy shader and if you want to bake the reflections into the texture that would be different.