I did mine I sanded the whole door down to natural wood taking off all paint then I used a penatraiting stain and all the natural wood grain cane out it came beautiful.
Scrape any old paint and clean the door. You didn't mention the condition or material or whether it would remain hung so here are some general rules:
It's easier if you can take it down but I've done both ways.
Once you get it cleaned up, tape off hinges, glass, knobs, anything you don't want painted.
Prime if needed.
Paint with an exterior door paint that is formulated for your type of door.
Highly recommend Sherwin Williams.
I'd recommend using a good bonding primer like this one, after you clean the door. Then you can paint with 2 coats of satin latex paint.
If you're painting the front door it's WAY easier to take it down. Prop it on some sawhorses.