If you want to volunteer, you can apply here. For off-campus and one-on-one chaperoning (field trips, reading buddies, and pizza supply shopping trips), you'll need a background check with fingerprint clearance. It takes a while to get going, so I recommend you get on that quickly if you want to participate in those activities.
Even if you did the application and fingerprint clearance last year, you'll need to resubmit the first four forms on that page each year.