Sometimes it can be hard to find volunteer opportunities if you are under 18. Volunteer Match is one website that tries to match volunteers with volunteer opportunities! When you search you just want to make sure to select Advanced Search and then click the box for "good for teens." You can narrow your search area and select what areas you are interested in volunteering for, like animals or the environment. It's smart to double check the postings to make sure you can volunteer if you are under 18 and to contact the host to make sure.
Check out: http://www.volunteermatch.org/