I primarily work with metal miniatures. Basically the process is like this:
1) Remove flash and scrape mould lines away with an exacto knife or small file.
2) Wash the figure in soap and water
3) Prime - I use Krylon primer, there are a variety of options for priming that all work well. Brushing on artist's gesso works well also. Some people just "prime" with a coat of black or white acrylic paints
4) Pain as per usual
5) For the armour, the easiest way is to paint it blak and then give it a heavy dry brush of a metal colour. Then go back and lightly do the most raised parts and the edges in an even lighter colour
6) finish the base as you like
7) seal with a matte sealer
For more info than you want:
http://www.faqs.org/faqs/games/miniatures/painting-faq/