Speaking from experience, start small and find out what you're good at. Even if it's as simple as helping a buddy move out of his home, you'll be surprised how good helping others makes you feel.
You talked about opening a shelter. Try volunteering at one first and see if you're cut out for it.
I know I get frustrated when I don't have the time to help out others, and having moved twice in the last 3 years, I don't have the same sense of community I used to have, but I'm working on it.
I have a friend who volunteers at a pro bono legal clinic twice a month. When I'm called to the bar, I'm definitely going to give that a shot.