Read One Second After or Alas Babylon or Patriots or The Stand. Those will cover all your questions.
In short, the supplies will run out. So you have to be prepared to hunt/fish/garden/barter and defend yourself. Staying alive becomes your job and it is a hard one. If you are responsible for keeping someone else alive, your workload doubles. If anyone takes prescription Meds to stay alive.....they won't make it. If a person is unable to take care of themself, they need to have a skill that others need so that they can trade. Otherwise, something very bad will happen to them.
If you are in an area where you have some neighbors close by, that has its advantages as well as some burdens. It may be handy if they all have guns and can defend a position or can help with a garden, but when your neighbor's Aunt Sally shows up after fleeing the city with her 7 nieces and nephews.......suddenly the neighborhood begins to fragment.
I could go on, but read the book(s).