Start with your home town (I can't look for you because I don't know where you're located) and look for a general volunteer organization. If you cannot locate one, check with your school's administration office; many schools now require their students to do community service hours as a graduation requirement so they may have a list of such organizations.
From there, you can look for organizations that are aimed specifically at helping children. You can pick something that reflects an interest of yours, such as sports; or something that means something to you, such as nutrition or medical issues.
An example of a sports related organization that helps kids is one that might collect and repair bicycles and then donate them to kids whose parents cannot afford to buy bikes for them. Another example is a group that wants to prevent kids from joining gangs so they provide athletic facilities where the kids can come together and have something to do other than run the streets.
An example of an organization that might help with medical issues is one that might collect caps and blankets for children who are hospitalized with cancer; or one that raises funds to help clinics buy medical equipment which will be used to treat low-income children.
I commend you for wanting to help; good luck in your search!
