Off Hiway 101 approxmately 10 miles south of Forks turn at the entrance sign and drive approximately 14 miles east to the visitors center. There are a number of hiking trails that lead into the forest. The Hoh Rain Forest is in Olympic National Park which is a Biosphere Reserve and a World Heritage Site.
Take rain gear.
The Moss Trail leaves from the Hoh visitor center and winds through the old growth forest. The Hoh is one of the wettest locations in the world averaging 150 inches of rain per year. I hiked the trail under an umbrella and photographed this image in a downpour.