Yay astronomy! As an officer of my school's astronomy club, I may have some tips to help you. In fact, you're probably just a quick Google search away from finding several interesting opportunities regarding astronomy.
First of all, does your school have an astronomy club? If not, starting one would be a great way to get involved with astronomy, or if there already is one, you should definitely join (and if it isn't too late, try for an officer position).
You should also try to Google any astronomical clubs/associations that are in your area. I know that there are several near where I live. This will give you the chance to join actual, amateur astronomers, learn more, etc; by paying a small, annual membership fee, you might even have access to dozens of telescopes!
For those last two things I listed, if you still really want volunteer hours, you could try setting up an astronomy presentation or a club for a local elementary or middle school.
Next, for getting involved, try to find astronomy-related places in your area and search from there. I have friends who volunteer at a local space museum. If you're lucky enough to have a NASA nearby (I don't know what the person who said that it's not going to help if you live in California, because I do and there's a NASA pretty close by to me) then I know that they offer internships/research and maybe even volunteering. You should check out their website. You could also just try a Google search: "Astronomy volunteer XYZ City" and who knows, you might find something really cool!
Hope this helps!