Read up on using the hypershade to create materials and textures. For metal, create a blinn material (or anisotropic for brushed metal) and name it. Double-click it to open up the material's attribute editor. In the color chooser, change it to a brown. In the specular shading section set the color to the highlight color you want (orange works) and overdrive the SV of the HSV so that the saturation and value are over 1.0. That should get you started with a gold material.

Then just select your doorknob, right-click on the material in hypershade -->assign material to selection and it should show up on the knob. You can also assign it by right-clicking on the object itself then go: materials --> assign existing material -->your material.