brings up a good point regarding woodgraining the door. Does your door have an embossed grain pattern? If yes, have you considered woodgraining it?
If you just want to paint the door, lightly sand the surface using 220 grit sand paper and finish using two (maybe 3 if necessary) a good quality acrylic latex semi gloss exterior or interior (depending which side you wish to paint). I wouldn't prime the surface since it's synthetic and non porous. If you want to woodgrain the door, lightly sand the surface and using a gel stain, apply the stain using cheese cloth allow ample drying time and then apply two coats of a good quality oil base spar varnish or three coats of a good quality waterborne semi or gloss finish. The exterior side must be finished using an exterior formulated varnish or paint whether oil or waterborne.