If you're looking to work with youth, Big Brothers Big Sisters of America might be a good place to start:
Volunteering - Volunteer work and volunteer programs with Big Brothers Big Sisters.
Churches often do a lot of volunteer work as part of their ministry, so if you're at all interested in joining a faith community, that might be something to pursue.
Additionally, there are community service groups out there like the Rotary, Lions, and Kiwanis. A lot of these groups have aging member populations, and are eager for new, younger blood. These groups have particular projects that they do on both a local and global scale, so if you're looking to do community service that helps at home and abroad, they are a good choice.