Rinse down the bench with a hose, and dry it off.
Pour a deck cleaning liquid that contains a brightening ingredient into a pump sprayer. Spray the bench from the bottom to the top. Use a brush to scrub the dirt off; use a small brush to get into small crevices. Hose it down again and let it dry.
Sand the bench gently with 100-grit sandpaper, just enough to even out loose grains, but not enough to remove the surface of the wood. Use an electric sander for most of the bench; sand small crevices by hand.
Wipe down the bench with a tack cloth, a slightly sticky cloth designed to remove all particles, dust, etc. before painting.
Apply gel stain with a small paintbrush to the sides and crevices -- coat flat surfaces with an applicator pad specifically designed for staining wood. Wait at least 24 hours before putting the bench outside or sitting on it.