Water is absolutely the most important thing that you need to collect and store for emergency situations. Just three days without water is enough to kill you. Water is also one of the cheapest things that you can collect and store and a great place to start.
Individual gallons of water are great and collecting them is not a bad idea, but they can be difficult to stack and they are difficult to store. While this plan is better than nothing, you will probably want a larger storage container.
Start out with a two week supply of water for each person in your family. You can continue to add to this supply as necessary. You can purchase 15 gallon water storage tanks. One for each person in your family should be a sufficient amount of drinking water for two weeks.
55 Gallon water barrels are another option. These can be found on E-bay and Craigslist for a fairly reasonable price. Some retailers are also selling these as water collection kits; however it is much less expensive to purchase the barrel by itself.
If you want to take it a bit further and you have the storage space you can buy IBC totes. These totes are used by companies to transport liquids. They are 270 or 300 gallon plastic containers encased in metal frames. When purchasing these it is important to find out what the company was previously using them for. Try to stick with containers that we previously used to store water or food. Avoid any that were used to store chemicals.
Proper storage of water is just as important as collecting it. If you are using water from your tap, and you live in the city, or you are using bottled water from a municipality (almost all bottled water is city water from somewhere), then you will likely not have to add anything to your water to keep it pure.
If you are using water from a well or other natural source, then there are some steps that you should take to purify your water. You will want to add Calcium Hypochlorite to extend the shelf life of your water. There are other methods of water storage purification as well, but Calcium Hypochlorite seems to work well for most people and it is readily available.
It is important to store your water in a cool dark place if at all possible. A basement or a closet are the best options. Direct sunlight is not good for water or the containers that it is stored in. Try to avoid areas that have a lot of windows or doors where sunlight enters.
While collecting and maintaining a good supply of drinking water is important, you should also think of the other things that you will need water for. While you generally do not need fresh drinking water for washing dishes and general cleaning, it is wise to have a supply of water to do these things. You will also need water to wash your hands, brush your teeth and shower.