Evaluate the condition of your deck. If the deck is reasonably well-maintained, you can probably manage the prep for painting yourself. If there are areas of mold and mildew, severe rotting or the deck is in very poor condition, you might want to call in a professional for advice and help.
Strip away any peeling or chipped paint. Making sure the surface is smooth and free of debris helps the next coat of paint penetrate the deck more deeply, keeping your deck looking fresh longer.
Sand down the surface of the deck. Paint adheres best to a porous (not a glossy) surface, so sanding roughs up the bare wood and helps the next coat of paint go on more evenly.
Wash the deck with water. As most paint cans indicate, paint works best on clean surfaces. Use a hose or power washer to remove all dirt and grime from your deck.
Allow the deck to dry thoroughly. Checking the weather beforehand is probably a good idea, since you probably need a few hours of warm, sunny weather to completely dry the deck.
Look over the surface of the deck again, removing any paint or debris that seems loose after the washing process. This process might seem tedious, but ensuring that the surface is as ready as possible makes the paint job much easier and more effective.
Apply a coat of exterior paint primer to the deck surface. Primer seals the wood from moisture, making your paint job last longer and maintaining the stability of your deck. Once the primer is dry, your preparation for painting the deck is complete.