Sanding your deck before sealing smooths out the surface of the wood to ensure an even application of sealant. It also means the sealant will be better absorbed into the wood. Your choice of sealant, stain or paint will depend on the wood type of your deck and the level of protection needed, but all kinds of sealing should only be done after sanding.
After washing, let your deck dry for at least 48 hours before applying sealant. Use a pole sander and 80 grit sandpaper, and avoid using a power sander. Always wear safety glasses and a dust mask when sanding.