Painting is a tricky process, especially with doors.

Ideally, the door should be removed from the hinges for easier painting, and should be allowed to dry COMPLETELY before being reinstalled (interior doors)

For exterior doors, paint on a gorgeous day, so the door can stay in the open position as long as possible, and pay attention to the paint manufacturer's recommendations on what temperature to paint at, as the time of year when weather dips below freezing is not a great time to paint.

Also, make sure you prep the door with the proper primer and use the right PAINT for the door!
Different paints are formulated for different surfaces, so ask for advice at your paint shop for the best type of paint to use, as well as what type of primer. You may likely need to sand the door beforehand so the paint and primer have better adhesion, but again, depends on whether you are painting wood, metal, time of year, oil vs latex, etc.