Water is probably the most important item to make sure you are storing to be ready for an emergency. We can live longer without food than we can without water! It is also important for sanitation purposes.
At the very minimum, you should store 3 gallons of water per person. This is meant to be part of your 72-hour kit and should get you through 3 days. It is better to plan on a 2-week supply of at least 14 gallons per person.
Many families have enough in their hot water heater. This water has already been treated and is very safe for drinking. Everyone should learn how to shut the water heater off from receiving new water in case the source is contaminated during the emergency. You should also learn how to drain it to access the water.
If your water heater is not large enough to hold what your family will need, there are many other options for safely storing water.
A quick search on the internet will lead you to appropriate information. This blog post is not meant to share all the details for proper storage, it is meant to encourage and inspire you to do something to build your supply.
In addition to the water that we store here at the house, we also have a backpacking water filter that will allow us to filter water from the nearby river and ponds. We store a large package of coffee filters as well. The backpacking filter will remove all the harmful bacteria, but it will not remove the floaties. By straining the water through a coffee filter it will be easier to drink.
For more on how we store and rotate water, you can check out my post from last Spring.
FEMA also has some good information on their page here.