New here

Soak it in bleach for a day or two, rinse well, let it dry, spray it with adhesion promoter and start painting. If u use good paints and a laquer based sealer your paints will not rub off. Good luck.

Yeah, what Bobtheodd said and welcome mate, any problems don't hesitate to ask.

There is no one way to do this, some paint while the figure is in pieces, some paint once assembly is complete.
Wash it first, this will get the factory release agent off the plastic. I use Krylon fusion spray paint as a base, then continue from there. Check the tutorials for more tips:
http://www.figurerealm.com/customtutorial.php
Oh, and Welcome to the Realm.