There are a few things to consider when storing bottled water. The first is temperature. Water should be stored at a cool, dark location to avoid it from getting too hot or too cold. Second is light exposure. Too much light can cause the water to taste funny and possibly damage the bottles. Third is humidity. Bottled water should be stored in a dry place to prevent the growth of bacteria or mold. Lastly, it's important to keep bottled water away from chemicals and other contaminants that could leach into the water and make it unsafe to drink.
The best place to store bottled water is in a cool, dark basement or closet. If you don't have access to a basement or closet, any cool, dark room in your house will do. Just make sure the room isn't prone to fluctuations in temperature or humidity. And if possible, try to store the bottled water on shelves rather than on the floor where it could be more susceptible to contamination.