The Department of Environmental Protection (DEP) has jurisdiction over properties that it leases out (to the Department of Sanitation, for example) or that are the roofs to water tunnel sites. If the lot you've found is managed by DEP, a good first step is to call Sarah Pecker, who is in charge of DEP's community garden agreements: (718) 595-5487. Her email is firstname.lastname@example.org. Be ready to take notes on your call and get the name of the person you speak to.
First, ask if they have plans for the lot. If they don't, ask if DEP might be willing to sign an agreement with neighbors who can steward the space.
After you finish with your call, record your notes on the lot page so that others who are interested in this lot can benefit from what you learned. We're all organizers together.
Some letters of support from elected officials is also helpful. You can use the "Political Boundaries" section on the lot's page to see who the elected officials and community board are for the lot.