You must be at least 16 years old to work as a construction volunteer. (16 and 17-year olds must have a parent or guardian sign their Volunteer Information Form and Waiver). Volunteers under 18 are not permitted to use power tools, climb ladders, or work on scaffolding or the roof.
Remember that you are going to be doing construction work so dress appropriately. This includes wearing sturdy hard sole shoes and avoiding loose or baggy clothes. Don't forget a good hat and sunscreen.
If you have work gloves, knee pads, a hard hat or safety glasses, bring 'em along. Same for basic tools like a hammer, nail apron, tape measure, etc.