You need three things for a blizzard to form: Cold air at the surface, moisture, and rising warm air. With these three things, you get a blizzard. The warm air rises over the cold air that's on the ground, but for it to make a blizzard, it has to be really windy. The cold air mixes with the warm air to make a blizzard.
To prepare for a blizzard, people usually stock up on food and water because they don't know how long the blizzard will be. It could be one day or it could be five days and people would be restricted to their houses and they would need food and water.
One of the strongest blizzards in the United States was called "The Great Blizzard of 1888." In that blizzard, there was about 20-60 inches of snowfall, and sustained over 45 mph. There were snowdrifts 50 ft tall. Railroads were shut down and people were restricted to their houses for over a week. This blizzard killed more than 400 people.
Another strong blizzard that was closer to now was "The North American Blizzard of 2005." This blizzard attacked many places in the United States, only dropping 12-15 inches of snow.