Paint stores have a detergent for grayed deck surfaces. I usually use
Porter Decktergent, but I'm sure the stuff at other paint stores are
good. An old push broom works well for scrubbing.

Or you might first try simply pressure washing with water. That works
if the weathering hasn't been going on too long. You just have to be
methodical -- use a fan tip and hold the sprayer so you're cleaning the
whole width of one board. Otherwise you can get an uneven cleaning,
and stainging will look bad.