Advocacy is defined as any action that speaks in favor of, recommends, argues for a cause, supports or defends, or pleads on behalf of others. No matter what you advocate for, the basic principles remail the same-evaluate what your goals are, collaborate with those who agree with your position, and educate others about your position. You can go as near as your family and/or neighbors, or as far as lobbying your senators and representatives to help secure funding for your cause, or anywhere in between.
Many times we think (or at least I do) "I'm just one person, what can I really accomplish?" Believe it or not, you can accomplish just enough, no matter what you do. Even if it's just telling one person about your cause and it makes them think about the importance of it, you've just increased the awareness of your cause.
Besides advocating for NASA and STEM funding and reduction of light pollution, I also advocate for military veterans, donating to charities such as Disable American Veterans and the Wounded Warrior Project.
If you're on my site, I'm assuming you have an interest in space-I highly encourage you to look into the various organizations that help us learn about, protect, and innovate for our skies and stars.