Blender 3D: Noob to Pro/Every Material Known to Man/Hard Plastic
Plastic is EasyEdit
Hard Plastic is made by taking the standard material (the material created when you hit the "Add New" button) and taking the "Hard" Value under the "Shaders" tab and turning it up to about 12. You can also turn up the "Spec" Value, also located under the "Shaders" tab.
Like every other material you can add scratches by making very thin normal maps and very lightly applying them, and maybe make the plastic reflective by turning it to about 0.05.