How to paint a tank depends on your intentions.If you want the interior painted or even the space behind the track-casing,its better
to do before assembling.When its only for the game,glue the hatches closed and paint it then.
You mentioned to have it already assembled,so i suggest to paint the space behind the tracks first in black or tin bitz.
Then paint the whole tracks in metal colors (prime black or tin bitz,then boltgun metal,add dirt or rust as you please.)
Go for the the other parts from inside out. Start with any recess,grills,gaps in darker color.Use your main color at next.
Finally do as much for shadows or scratches as you feel able to.

look for Modelling and painting articles!