Find out what the school district policy is on volunteering at school functions. Most school districts require you to fill out an application and get approval before you can start to volunteer with the kids. Go to the school district's website for the policies and volunteer application.
Know the rules. You should be familiar with the policies for the school where you intend to volunteer. The school may even have more specific rules for field trips.
Find out when and where the field trip is and at what times you will be needed to chaperone. You may need to be early to help pack up or stay late to help unpack or clean up. Be sure to wear comfortable shoes and appropriate clothing for the destination.
Do what the volunteer coordinator or teacher in charge asks you to do. They may want you to ride on the bus and keep order, or to drive your own car and bring the sack lunches and drinks.
Find out from the volunteer coordinator or teacher in charge what you should do if you see a child misbehaving. Sometimes you can just tell a child to stop doing what he is doing or you may need to get a school official to handle it.