I had wanted to do something like that for years, but was unsure of how I could actually help. It turns out that you don't need any special skills to help someone. All you need is a little time. Working at the battered women's shelter was a great experience. I got to see how just being present meant the world to someone who didn't even know me. Working at the food pantry was another eye opener and gave me a different perspective on things.
I also gave blood for the first time at Lifesource. That was pretty cool and didn't take very long.
Check out www.idealist.org for a nationwide search on volunteer opportunities.
Goodluck!