First you'll need to show that there is community support of a broad nature for the
project. If it's just you, the gov't won't go for it. They need to see that it will be funded, installed competently, maintained for many years, and deeply supported and enjoyed by the community. They will want to see a track record showing you or others involved in the project have done food forests that are successful, so pick a good team of experienced people. Lots of places have done this, and there are good guidelines for doing it. The Beacon Hill Food Forest in Seattle, which gets a lot of press even though there are many other worthwhile public food forests, has gone through the whole process very successfully and you
should contact them or other groups that have done public food forests (and there are dozens now).
Good luck!