Very good advice.What is the door made of? Wood, metal, fiberglass?
When painting a door, pull the weatherstripping out of the door frame. It is usually held in place by a spline. This will allow you to close the door and let the paint finish drying over a day or two. Especially important if you use oil based paint. If you leave the weatherstripping in place it could become glued to the door, been there, done that.
My fiberglass and metal doors are painted with gloss latex paint. On a metal door you need to remove any rust and prime it before putting on latex paint or it will rust. Automotive spray primer works fine. Latex is more forgiving than oil. Oil is probably a more durable finish. Clean the door, lightly sand any gloss off the door, clean it again and let it dry. Then apply your finish coat after priming any spots that are bare metal.