If you are going to use chlorinated water from a municipal water supply (like the taps inside your home) to fill your water storage containers, you do not need to pre-treat the water. Make sure your containers are washed and rinsed, then fill with water and store.
If you are going to use non-chlorinated water, or water from your outside spigots (garden hose), you will need to treat it with bleach. Use standard liquid household bleach (5-6% sodium hypochlorite) without additives or scents. Add 1/8 tsp. bleach for every gallon of water.