Resources can be gathered from all over.

FOOD & WATER- Use scavengers and farmers to keep up the basic supplies. Research preservation to keep it all from going bad.
WOOD & METAL- Other resources are used to build structures and fortify areas.
FUEL- Used on vehicles and some construction projects. Find enough and you can leave the city once it has been picked clean.
AMMO- To get the guns working, but always keep a few machetes around.