To clean and properly seal a deck, first sweep the debris — leaves, twigs, dirt — off the top. Pay particular attention to the spaces between boards. Leaves will rot in there and rest on the joists, eventually rotting the boards. Use a putty knife to clean the crevices.
Clean the surface of the wood. There are several products on the market, or you can use bleach, cut half and half with warm water, and a scrub brush. This is ideal for small areas and will work wonders on algae and mildew and the like. (Straight, undiluted bleach is not advisable for a deck, and it should not sit there for any length of time.)
For larger areas, rent or purchase a pressure washer. Spray the bleach/water mixture on the boards first and let it soak in for a while before pressure washing. Make sure you cover up shrubs and plants before you start spraying bleach.
Once your deck is clean, apply a redwood stain or a waterproofing deck sealer, which will repel water and make the deck last longer. To apply the sealer, purchase a pad with a little reservoir and brush it on. Or use a foam roller. Go over the entire deck surface with the sealer