I have a new wood mantle that has a layer of laminate and a varnish finish. How do I prepare the wood to be painted and what kind of paint should I use?

You will need to sand it with fine sandpaper and wipe off with a damp cloth. Then you will want to prime it with an oil based undercoater but not one of the quick dry primers as they dry to brittle and then finish with an acrylic/latex semi-gloss trim paint.