I used to cook regularly for a soup kitchen. When I was hugely pregnant with The Offspring, I got light-headed in the heat and had to stop. For years, I told myself I'd go back. But a full-time job plus child care turned out to be a more challenging combination than I ever dreamed, and while certain things about parenting have gotten easier, it still demands a whole heck of a lot of time. Okay, so I give up. I'm not at the soup kitchen any more.
Meanwhile, though, I've been blogging, an activity that can be done in much smaller chunks of time. I think about issues I want to blog about on the train, on the bike, in the kitchen, in the shower. I can grab a few minutes between work or family responsibilities to write a few paragraphs, even a few sentences. Sometimes it takes me days to finish a post, and that's okay.
Eventually, The Offspring will move out, and different kinds of time will be freed up again. And eventually again, I'll retire. At the points of transition, I'll have the opportunity to rethink what I spend time on. For now... I do what I can.
And now I'm talking to you, dear reader. Yes, you can make a difference. But how? You're just one person, with limited impact.... Margaret Mead has been quoted so often it's more or less doctrine: "Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it's the only thing that ever has."
Find your cause. You need a cause you're truly committed to, because sometimes it will feel like you're the only one. Your commitments may change over time; that's okay. The world changes, and as you learn more, you'll see different facets of it that will seem less or more important.
Find a group of people who share your views. Depending on your temperament, you may do well with a traditional face-to-face community, or you may prefer to interact with people on line.
Study your cause, and keep studying, keep learning more. There's always more to find out, more knowledge being created. Become the creator of knowledge.
Figure out what you have to give, now: time, money, a skill. Later, you may have more money, less time (or the other way around), different skills, and as you keep learning and re-evaluating, you may be in a position to help differently. But don't wait until you have [x]. Do what you can, NOW.
That said, if constraints on time or money limit your availability to help now, don't beat yourself up.
Do what you can. Don't give up.