A nuclear attack: what the official guidance says to do

The three words

Get inside, stay inside, stay tuned (Ready.gov).

If you're outside when it happens, lie face down to protect your skin from the heat and flying debris. When the blast wave passes, go inside.
Inside, go to the middle of the building or the basement, away from windows. Close windows, doors and fireplace dampers, and turn off fans and ventilation that pull in outside air.
Stay there for the first 24 hours unless officials tell you otherwise. Listen for instructions on a radio.

Why staying put works

Fallout is most dangerous early, and it fades fast. Federal planning guidance uses the 7–10 rule: for every sevenfold increase in time after the detonation, the radiation rate drops about tenfold (Planning Guidance). Seven hours after, it's about a tenth of what it was at one hour. Two days after, about a hundredth.

That's why the guidance says the best first move is to shelter in the nearest, most protective building, expect to stay at least 12 to 24 hours, and not leave before 24 hours unless told to. Trying to drive out early can mean driving through the worst of it.

What counts as shelter

From the same guidance (Planning Guidance):

Good: a house with a basement, the middle of a large multi-story building, a parking garage or a tunnel. These cut the fallout dose by 10 times or more.
Not enough close to the fallout: a single-story wood-frame house without a basement, or a vehicle.

Knowing now where you'd go is the most useful preparation there is.

Power while you stay inside

This is where your own power matters:

A generator has to run outside, at least 20 feet from windows and doors (Ready.gov), and going out to refuel it is exactly what you shouldn't do in the first day. Batteries indoors don't need you to go out. Solar refills them later.
A radio is the lifeline: "stay tuned" means a battery or hand-crank radio. Keep a spare wrapped against EMP, which CISA's simple method covers (CISA, more on EMP).
After the first days, a nuclear event may mean a long grid outage. That's the grid-down plan, and water and food for two weeks or more (Ready.gov, FEMA).

Common questions

What should I do if a nuclear bomb goes off?

Get inside, stay inside, stay tuned. If you're outside, lie face down to protect your skin from heat and debris, then go inside once the blast wave passes. Go to the middle of the building or a basement, close windows, doors and dampers, turn off ventilation, and stay at least 24 hours unless officials say otherwise.

How long does fallout stay dangerous?

It fades fast. Federal planning guidance uses the 7–10 rule: every sevenfold increase in time after the detonation cuts the radiation rate about tenfold. That's why the first 12 to 24 hours inside matter most.

What's a good fallout shelter?

A basement, the middle of a large multi-story building, a parking garage or a tunnel. These cut the dose by 10 times or more. A single-story wood house without a basement, or a car, isn't adequate shelter close to the fallout.