Attend a meeting. After you find a group you think you might be interested in working with, find out when they meet and go to a meeting. This is a great way to learn more about the group and the people in it.
Going to a meeting will give you a better sense of whether this group is right for you and if you want to be involved in the types of work they do to combat injustice. It's also a great way to meet people who feel the same way you do about the issue, which can help you feel more inspired to take action yourself.