The safest place to be is in a building, preferably a basement or central room with no windows or outside doors. Bring your pets in too.
Vehicles do NOT offer adequate protection - if you are in your car, get into a building as quickly as possible.
Avoid doors and windows. If possible, stay away from rooms which border upon outside walls or the roof, because that's where the most radiation will collect.
Shut off air conditioners and fans. Close all vents to the outside.
If you or anything you brought inside was exposed to radiation, you need to decontaminate.
Think of contamination from radiation as being similar to dust - it settles on exposed surfaces (clothing, skin, hair, objects) and can be tracked around and contaminate other people or objects it comes in contact with.
If you were outside and exposed to radiation, immediately remove your outer layer of clothing CAREFULLY (if you can), and place in a plastic bag. Wash any exposed skin or wipe with a damp towel or cloth, put the used cloth in a plastic bag to avoid contaminating other things with it. Put on clean clothing, if available.
You can use tap water or well water for decontamination, even if it is not safe to drink.
If your pet was outside, wash them with water and shampoo and rinse thoroughly. If you can, protect yourself with rubber gloves and a dust mask while doing this. Re-wash your own hands and face after bathing your pet.
Most radiation will dissipate or settle within hours. Until then, the best thing to do is to stay where you are.
Close all windows and doors, vents, fireplace dampers and any other openings to the outside.
Drink only bottled water/juice and food in sealed containers (boxes, cans, jars, packages).
Food that was in your refrigerator or freezer is safe from radiation, as is food that was in a closed pantry, closet or drawer. Pet food in closed containers will also be OK to use.
Wipe containers with a damp cloth before opening, dispose of cloth in a plastic bag. Wash or wipe counters, dishes (your own and your pet's) and utensils with a damp cloth before using and dispose of the cloth in a plastic bag.
If you were exposed to a large amount of radiation and develop signs of Acute Radiation Syndrome (burns, nausea, vomiting), seek medical attention as soon as it's safe to leave your shelter.
A battery powered radio or web-enabled device will help you keep updated. Phones may or may not be working.