Depending on how bad your deck is and what look you are trying to achieve, you have a couple options.
The first and easiest is to rent a pressure washer and really clean that deck off good. You would be amazed at how good your deck can look from a pressure wash! When you're done, let it dry thoroughly then treat it with a good oil base treatment. Oil base will last longer than a water base treatment.
Your other option is to use a cleaner like the Behr 15-minute cleaner. It's really caustic and you need a LOT of care when using it, but it does an excellent job of cleaning and preserving your deck. Don't let the name fool you-it will take a full afternoon or more depending on the size of your deck to do a full cleaning, but it will add fresh life to an old deck.