Sit in the front if you can. Bring a mat with you. You will find good seats when you go early. Starts at 5pm (music at 6pm) and ends around 9pm. It gets fairly crowded during the first hour. There are vendors around the street on the library side. Average cost is $5. You'd save money by packing a small snack. Hope you have fun!
I usually like a spot near a sidewalk for easy access and good people watching. I would avoid the areas around the playground equipment unless you want running children and flying sand to be a part of your JitP experience.