For starters, you could define what type of metal/plastic you're looking to make.
You need a basic understanding of what each channel in your shader is for. With that and carefully looking at a real world reference, it will start being easier putting 2 and 2 together.
Here's an example: you want to make a gold shader.
Most people go about gold totally wrong, they see it as yellow, but it's actually not. For gold, you would pick a dark brown base colour and yellow is what you would put in the specular colour, the specular colour channel is where metals get most of their colour.
Another thing you have to think about is how glossy is the surface. Reflective materials like shiny metals or plastics have a smaller tighter specular highlight. Dull plastics have a bigger softer specular highlight. Brushed metals are anisotropic, they don't have a round highlight, instead they have a long one going against the brushed grain. Always remember, that even dull surfaces have a tiny bit of highlight and reflection.
And then there's another thing called fresnel. If you look at a linoleum floor, directly underneath yourself, you won't see your own reflection, but if you look further, you will see objects reflected on the linoleum.
There is nothing that will help you understand this better than looking at real world objects and how they interact with different sources of light.