See the Habitat for Humanities web site for more information. Call the national office or your local office for information about volunteer programs in your area. Many state parks offer volunteer programs, and in these programs you can try anything from educational programs to trail construction and maintenance. This site for the North Carolina State Park system shows some of the possibilities available. Contact a state park near you and see what options are available if you are interested.
Most large cities offer a wide range of volunteer opportunities.
This site for the city of San Diego shows the different programs in San Diego. Any city will offer a similar set of programs. Look in the phone book and call around to see what might be available where you live. Many city governments are large and fairly disorganized. When you think about it, reading is one of the most important skills an adult can have.