Well, for starters don't paint it, stain it. Paint lays on the surface and with heavy foot traffic & weather/sunlight a horizontal surface takes a beating & paint just doesn't hold up on decks. Also, do a search, there are 7 or 8 topics on here right now about refinishing a deck. A semi-transparent stain is the best option for a deck..........many companies to choose from such as Sherwin-Williams, Benjamin Moore, Sikkens, etc. Make sure you GENTLY clean the deck with a soft bristled brush or a low setting on your pressure washer, let dry for several days, and then evaluate the deck. If it's splintering or has left "fuzzies" on the surface, you may have to sand to remove that material and get down to a sound surface. Clean up the dust and you can begin staining.........of course all this info is worthless if you have a failing PAINT film on the deck.......in that case a complete stip of the surface is gonna be required and you may want a pro to tackle that one.