Search the Internet for other groups in your area that can share their experiences with you. Clean the site. Wear protective gloves and dust masks. Develop the design. Begin to order materials.
Cities usually have zoos, and can provide natural fertilizers.
Often, plant suppliers or lumber yards will donate materials. Glove manufacturers are quite willing to provide gloves to schools or educational groups that are involved in starting gardens. Many seed companies will donate free seeds to community groups who have gardens. It just takes a little sleuthing to find free sources of supplies for your gardens. Decide on plot sizes.