This should get you through your first event, but you're always welcome to bring more than what's listed here.
Clothing
Outdoor Gear a chair or blanket to sit on (avoid overtly modern designs/logos/materials)
Food and Drinks
Overnight Supplies
Medical Supplies
Other things people bring
You may already have some of these things. If you don't, you can often make, borrow, or buy them. Reach out to your local Hospitaler for help!