Okay, as for chipping, a good plan is to use a dark non metallic colour first, and then paint inside of this with your chosen metallic colour. I usually use Scorched brown, since I find black to be too dark. Then chainmail for the scratch or chipping itself, and then badab black over this, to give definition.
I´ve used this technique on these two models. Mind you, the walker is not finished yet, still needs some more weathering and funny stuff.
For more tips, I´d recommend Grey Death´s excellent article here:
http://www.dakkadakka.com/wiki/en/Tank_Weathering
Although there are some variations to the techniques he mentions, these will give you excellent results.
Hope that helps.