When I did a couple events with habitat for humanity, I met a number of people highly involved in other local charities.

I think volunteering is a good way to meet people who regularly volunteer. That's the crowd that is likely to know what all is going on in your area. Learning about organizations through volunteers will also give you a good idea for which groups are actually working with their community directly and which ones are more talk and less action.

So I would recommend finding a group to volunteer with to get started, and you can switch to causes that more closely align with your interests once you start making connections. Habitat for humanity, food banks, local environmental cleanup days are all easy to find just about everywhere.