People can live for days without food, but not without water. It is essential to maintain life, so water storage should be one of your highest priorities when establishing a pantry.
Plan on at least a gallon a day per person to provide drinking and washing needs. I am talking about dampening your face and brushing your teeth; not real bathing.
First things first, let's talk about containers. They must be food grade, preferably opaque and not biodegradable. Milk jugs and bottled water from the store are in containers that will begin to break down after a few months. If I do purchase water before storm season I store it in an extra shower that we have, not near my food supplies. I found white, opaque, five gallon, food grade water containers at my hardware store. Considering that a gallon of water weighs about 8 pounds, it would be difficult for me to maneuver anything larger. There are also specialized containers available online, it depends on your individual needs and your budget.
When storing tap water it is important to treat it with household bleach. Just plain bleach, not lemon scented or soapy. Add 1/8 of a teaspoon per gallon of water. Tap water is not sterile by any means, so be sure not to skip this step. Your water should last for several years if treated this way.
Water that is stored for a long period of time can go "flat". This means that it is no longer oxygenated, but it is still safe to drink.
It is always a good idea to stock up on toiletries and other essentials (to your family) that you cannot make for yourself. Just figure out what your usage is per month, multiply by 12 and start shopping wisely.
When preparing your pantry for an emergency situation (eg. hurricane season) it is always advisable to keep two extra weeks of medication on hand. The emergency management departments suggest this every year, but good luck convincing your doctor and insurance company. I always manage to stockpile a small amount of my medication by ordering two days early each month. This way I am acquiring two extra pills each month and after seven months I have my emergency supply.
The subject of what to store in your pantry is endless. Your supplies should be based on your personal preferences and needs. Keeping your pantry supplies up to date is a constant process and should be a part of your weekly/monthly shopping trips.