Find a shopping mall and hope it was closed during the outbreak, and then build a moat (minus the water) around it. Then if you can, steal grenades from wherever you get grenades, and throw them down in the moat where the trapped zombies are because they're stupid and don't realize the purpose of a moat is so they can't get into the main defense of the living people so they fall in the moat and get stuck there where they can't get out and they just run around making zombie noises like idiots because they're zombies.