First, remove all of the door furniture (handles, locks, letter box, house numbers etc) Next, fill any dents or holes with a good quality wood filler. Let the filler dry completely before you start to rub it down. Ensure the surface of the door is dust free. You can use a TAC cloth to remove any dust from the door prior to your painting.
Applying an undercoat will improve the final finish and provide a good surface on which to apply the topcoat(s). It may be necessary to apply two coats to adequately cover the primer or wood colour.
Before you apply your top coat you might want to key up your surface with a 120-150 grit sandpaper. Be gentle as any heavy scratches might show. More than one layer of gloss may be necessary to give a smooth high quality finish.
Not confident you can get the professional look, then why not give Holloway Painting & Decorating a call for a fast, competitive quote!