The quantity of water you will need to store depends upon how far away from reliable water sources you plan to travel. For personal use 1 litre plastic or steel water bottles are fine or you may consider a metal vacuum flask, which is not only almost indestructible but can be used for hot as well as cold liquids. Another good idea is water bags, which come in many different sizes and, to save space, can be folded away easily when not in use (Ortlieb are the best). Larger ones can be hung up to wash your hands under or even to use as a shower.
If you need to carry larger quantities of water then 20 litre (5 gallons) black plastic jerry cans (they are black to help eliminate the growth of light-loving algae) are a good idea and can be obtained through most military surplus stores. Stored on the roof rack (ideally you should try to store heavy kit as low down and central as possible) or in specially designed racks they are a robust and reliable method for carrying water. If travelling in a larger vehicle it is worth considering a single large water tank. Built from moulded plastic, fibreglass or aluminium they can be fitted with gravity fed taps or electric pumps and save the heavy lifting involved with jerry cans, but it can be more difficult to estimate your remaining reserves than when using jerry cans. When mounting a large water tank the best place to least effect the vehicles performance is low down and between the axles (behind the drivers seats is ideal), this may not be the most convenient for access so you will probably need to find a compromise.
Always take at least one jerry can as a backup to use when your water source is hard to get at.