Do NOT be deceived by the full shelves you see at your grocery or pharmacy. Modern U.S. history has demonstrated repeatedly that in case of pending emergencies people rush these stores and the shelves can be emptied within a day.
The two main items you must provide are food and water. FEMA has determined that a family of four needs a minimum of 28 gallons of water for one week - 53 2-liter soda bottles. That figure assumes some frugality of usage - mostly for drinking - and that users are staying in one location with minimum activity. More activity by any individual may require more water consumption.
If water is to be stored for any length of time, some containers will be required and some preservative should be added. Prepper suppliers such as The Ready Store sell water containers from a few gallons up to 50 gallons, and also sell a preservative, although a few drops of bleach might suffice for preserving the integrity of drinking water for months.
Food is less critical, except perhaps for children. Again, the more activity required for survivability, the more food and quality of food will be required. Protein is essential for meeting muscle energy requirements, but carbohydrates are converted to sugars which also provides energy for the body. For short term food storage there are plenty of shelf items at the grocery store that will suffice: canned goods, sealed bottled items, dried goods. Most will list a date of usability on the label, although many are good for much longer than label dates; good judgement is necessary. For example, a good all around general food is peanut butter, which stores well for a few years; obviously not indicated for those with peanut allergy.
Another consideration for some are medicines. It is possible that many will not be available after a few days in an emergency situation. I take advantage of the insurance allowing me to refill a prescription a week before I run out. I take the left over Rx and store them in a dark glass container with a seal under the lid; adding wax paper under the lid can improve that seal. Many meds, especially tablets, are viable quite some time after the expiration date; expiration dates are determined by lawyers in order to protect the manufacturer in unusual cases and circumstances. Again, some judgement is required.
Keep some cash on hand; ATM's may not be working.
One last immediate need might be vehicle fuel: gas or diesel. A few 5-gallon containers are useful but storage can be dangerous; beware of any fire hazard... fuel can be explosive! A good container will seal fumes from escaping. Some preservative is needed for storage for more than a few weeks. I use Sta-bil to preserve gasoline; it is avialble at WalMart or auto supply stores. Place a label on the container's handle to indicate when the fuel was stored and if the preservative was added.
I somewhat recently finally cycled through remaining stored food items from about 1980 - a bit slow responding. The remaining canned oatmeal had taken on a taste and I disposed of it. A few days ago a carton of six #10 cans, three with oatmeal, arrived from the Ready Store - good for 25+ years of storage. I just have enough storage space remaining for these 6 #10 cans.
I have about 1 gb in survivability files covering a wide variety of subjects. I could provide the bulk of those on a thumbdrive {some of it copyrighted material} if anyone is interested. Subjects range from communications to EMPs and Faraday cages to camouflaging stored items; food, fuel, bio-waste, on-and-on.
The two main items you must provide are food and water. FEMA has determined that a family of four needs a minimum of 28 gallons of water for one week - 53 2-liter soda bottles. That figure assumes some frugality of usage - mostly for drinking - and that users are staying in one location with minimum activity. More activity by any individual may require more water consumption.
A few years ago DH had to do something with water pipes/well/something or other and said water would be off for several hours shortly. So I filled up every single available bucket/jar/bowl/pot etc with water as fast as I could. Turned out he couldn't get water back on for at least 1 1/2 days IIRC. We were totally frugal, cooked minimally, washed very few dishes, re-used water (first for hands and face, then same water for ickier stuff, finally for flushing toilet), took sponge baths.
I figured out how much water 2 people used in less than 2 days (or thereabouts) for very minimal water use and it was around 42 gallons.
FEMA's pathetically minimal water storage means they can drink and use a few drops on their hands and face and that's about it. Barely any washing, bathing, flushing toilets, cooking etc. They should make it clear that people should store much more water than that.