Select a color for your door by looking at the exterior of your home. You can choose a color that matches the rest of your trim, or a color that complements the house and trim color. Since the door is new, you may need to paint the inside as well; some doors come prefinished, while others do not. Choose a finish as well; exterior paint is available in gloss, satin or flat finish, and each will give your door a slightly different look.
Open the door and slide a drop cloth underneath to protect both the inside and outside of the doorway. Remove any existing hardware, including the doorknob and knocker. Most new doors will come without these items, but they should be removed if present. Use masking tape to cover any windows or trim you can't remove. Slip a piece of cardboard into the doorjamb to prevent the door from closing as you work.
Stir the paint and pour it into a paint pan. Use a foam brush to add a coat of paint to the top, side and bottom of the door. Painting these surfaces will help your finish last longer, and extend the life of the door as well. Use a paint pad or roller to apply primer to the outside of the door, beginning at the top and working your way down. Don't overload the roller with paint, and go back over any drips quickly. Let the first coat of paint dry completely, then add a second in the same way. Allow the door to dry overnight to give the paint a chance to cure.
Check the door out in the daylight to make sure you didn't miss any areas; touch up with a soft paintbrush as needed. Clean up any drips or spills with a solvent-based cleaner, then install the doorknob, knocker and any other hardware you have for the door. Remove the drop cloth and other painting items and discard them in the trash.