When someone messages me in a panic asking where to start, my answer is always the same: water. You can go weeks without food. You can only go a few days without water. And in most emergencies, clean water is the first thing that gets hard to find.
The good news is that water is also the cheapest thing to prep. You don't need to buy a single bottle to get started. Wash out some 2-liter soda bottles, fill them from the tap, write the date on them, and put them in a closet. You just started.
How much you need
Plan on one gallon per person per day. That covers drinking and a little bit of cooking and washing. A family of four needs about four gallons a day, or 56 gallons for two weeks. That sounds like a lot, so build it up a few containers at a time.
Have a way to treat it
Stored water runs out eventually. Keep a bottle of plain, unscented bleach and a good filter so you can make other water safe. The Water page has the exact amounts.
Start this week. Fill a few bottles. It's one less thing to worry about.