The best idea is to go to Ready.gov
and the Red Cross website
to get an idea of what the experts say is the bare necessities. Also, seek out your local county or city disaster preparedness website so you know what risks are out there. If you live in a flood plain your preps might be substantially different than if you live on a major fault line. If you''re worried about a flu pandemic, you'd better put up some gatorade, but if you're worried about a severe winter storm, that's a waste of your time right now. No matter what, the Red Cross suggestions of flashlights with spare batteries, chem lights, first aid supplies, battery operated radio, medications, hygeine items, and other essentials are a good place to start.
No matter what, the basics have to be taken care of. As a general rule, a person can live 3 minutes without oxygen or with major bleeding, 3 hours without shelter from extreme conditions, 3 days without water and 3 weeks without food. As a result, injury prevention and treatment should be a high priority.
Smoke alarms, carbon monoxide detector, seek out and minimize hazards in the home (no large potted plants on the shelf above your bed in case of earthquake, keep guns properly secured from children, etc.), get basic first aid skills, and keep a well stocked first aid kit at home and in the car. Do all family members know how to swim? Do you have an evacuation plan in case of a fire?
Next, be sure you can keep your family warm in the winter and cool in the summer even if utilities fail. For some this might be a wood stove, for others, a kerosene or propane heater, for some a swamp cooler with a back up power source. Natural gas is far more reliable than electicity in an emergency because the utilities are buried, but does your furnace or boiler work without power? Is there an alternative way you could power it (generator, solar, batteries & inverter)? Keep this in mind when planning for the worst. Also, if you live in a fairly mild climate, consider adding a few more blankets to you home or buying a few sleeping bags.
Water is a fairly significant issue for most people living in a small space. An average adult consumes a minimum of on gallon a day and that does not account for water used in cooking, cleaning or hygene. If there's four of you, that's 28 gallons for a week! However, you can generally drain your hot water heater after turning it off and obtain up to 40 gallons of water. Toilet tanks (not the bowl!) house another 1.6-3 gallons, but don't use it if you use sanitizing tablets. If in doubt, boil it for 5-10 minutes or filter it with a backpacking filter. If you live in a hot climate, double your water storage, not just for drinking, but also for wetting clothes for additional cooling.
While food is often people's highest priority, it's not essential for life unless you're in a cold environment. That said, it's easy to store and bring comfort and routine into your life that will ease stress in an emergency. Think about foods you like that require a minimum of preparation. Spaghetti is easy, cheap, and only requires boiling water and simmering. A two burner Coleman stove makes cooking easy, but needs to be done in a well ventilated area. Portable butane stoves produce less carbon monoxide and are safer to use indoors, but make sure you have enough fuel stored to last the duration of the event. In any case, buy things you like to eat, that have a long shelf life, and keep them in a separate cabinet. While dehydrated foods are super easy and convenient, they're expensive and don't taste like home cooking so it's just another disruption to your life in a time of chaos.
As for what you should be prepared for, personally, I think seven days is the minimum. That's seven days, fully self-supported with no outside power, water, or food. Beyond that, it's likely that basic service will resume in most localized disasters. If you're talking about the scale of something like a global grid disruption or a supervolcano, then you're talking about another league of disaster. That's where you need to seriously consider storing a year of food for each family member, a long-term secure water supply, alternative heat, alternative power, and a way to supply your own food after the worst is over. I wouldn't worry too much about Yellowstone waking up based on the latest
but if you are concerned about The End Of The World As We Know It (commonly referred to as TEOTWAWKI), then start with the basics and build food capacity from their.
For me, I try to be ready for most everything. I've managed to put away roughly 15 months of food including 3 weeks worth of freeze dried backpacking meals for traveling, 30 days of MREs because they're easy to prepare, and the remainder in wheat, corn, rice, beans, pasta, canned and freeze dried meats, veggies, butter, milk, cheese, and other sundry items. I have three years of seeds stored as well as the capacity to heat my home through the winter even with the natural gas down. With the gas running, but no electricity, I would have little disruption and a new solar array will mean nearly no disruption. But be aware, getting fully squared away to this level is a process of years, not weeks. I started prepping in 1986. If you want to get there, it starts with the basics. If you can't make it through a week off the grid, you'll never make it a year no matter how many pounds of rice you store.