September is National Preparedness Month. If you are new to having a prepared household, start by reviewing my 'Preparedness 101' page on this website. Visit some other beginner sites on the web as well.
Once your specific emergency has started, all you can begin to count on is what you have at the time. Make sure you have at least enough to get you through the reasonable expectation of time to restore normalcy. If your likely emergency is a power outage and it usually takes a day to repair, maybe have 2 days' supplies on hand -- stuff that doesn't require POWER to use. If all your cooking capability is electric, make sure you like cold canned food or have a BBQ grill AND charcoal or propane. Don't count on buying it after the fun begins, because several hundred or thousand people got 'there' first and bought it all.
Get the picture? For now, read and think so you can act and do. Take care of yourself and your family by thinking ahead.