I've never started a community garden, but the City of San Diego's
Community Garden page has an application form. They provide a phone number, (619)525-8213, which might be helpful in answering your questions.
The San Diego Community Garden Network sounds like a good resource and also a good place to register your garden so that other people can find it once it's up and running.
I'm interested in hearing advice from Permies with more
experience in this area!