volunteers.aarp.org/ Register For Free & Find Ways To Volunteer In Your Neighborhood.
www.uso.org/Support-Our-Troops America's Wounded Warriors Need Your Support. Join the USO Now!
www.cornerstoneondemand.com/ Reduce costs, improve quality, manage nonprofit. Learn More
www.rethinkchurch.org/ Make it about giving back. Find the true meaning of Christmas.
www.libertymutual.com/Responsibility Mentor Network Allows Kids to Get a Mentor to Help Them. Learn More.
How I did it: i first googled to find the organizations that like, and then contacted them offering to become a volunteer and attended my first meeting at one of them, and now i'm officially their volunteer, yay :D
Lessons & tips: just do it, don't wait around too much, just google the organization you want and contact them right away, no need to be shy, go for it!
Resources: the internets, google