There are three overarching categories in the way a person can contribute to a cause or a non-profit. Any single person can care about a number of causes and showing commitment to all of them can be difficult. Sometimes you have to prioritize your causes and how you can show your support for each will vary. Here are a few ways you can support a cause:
Work for it.
Volunteer or join the professional staff of the non-profit organization formed around your cause. If there is no 501(c) 3 to support your cause, start one.
Organizations need money to function. Go to an event. Make a financial contribution to an annual fund or a capitol campaign (for a specific project, like a building)--those annual funds pay to keep the lights on.
Awareness is important to making (impacting, if you like that word) change. When people who know you associate you with a cause, it becomes more familiar to them, more relate-able (assuming people like you).
Act on it.
This isn't an option for all causes. But if you support child literacy, you could look to read to kids at a local library; if you're passionate about the environment, recycle, compost, look for renewable energy. Take matters into your own hands.
Whatever you do to support your cause, do not diminish the contributions of others. If you're really excited or passionate about a particular issue, that's great, but not everyone else is. You hurt your cause when you attack others as not supportive enough or fake--who wants to help if they don't feel appreciated or if they feel like their effort isn't enough?