A good way to start is to make a list of everything you buy for a month - food, soap, toilet paper, etc. That will give you (assuming you don't eat out a lot) an idea of how much you really need. You can also see what fresh and frozen foods you buy, and which can be replaced with canned or dried.
Then, if money is a concern (and when isn't it), you can buy just a few extras each time you go to the store. For example, if you need a new tube of toothpaste, buy two - the extra $3 or 4 won't be as much of a hit as buying an enire months supplies, all at once. You just have to be disciplined enough to buy another when 1 is used, not when 2 are. Put the new items on the shelf in in the back, and rotate everything.
Now, you need a way of cooking and heating. Hopefully you have a fireplace, store a lot of wood. Close up most of your house and get everyone together in one or two rooms to cut down on heating requirements. Get sleeping bags, don't plan on using lightweight blankets or radiant heat.
Get a propane (easier to use and store) stove and a couple of 5-gallon propane tanks, and the hose to connect the two. You can often find these at garage sales. If it's at al possible, cook outside for safety's sake.
You do have a fire extinguisher in the kitchen, and smoke detectors, right? Your local fire station will most likely be tremendously willing to show you how to use an extingusiher, if you ask.
Water is an issue - realistically you'll need 2-3 gallons a day per person (more in the summer or winter) for cooking (especially with dried foods) and basic hygiene. If you don't have a large body of water handy you should store it. I have a couple of 55 gallon poly drums set up plumbed into the water softener, so the water is always refreshed. You can also get water out of your water heater (make certain that the valve actually works, they occasionally will corrode shut). If you have more than one bathroom, decide on which ONE you're going to use, and then save used water (after you've cooked, washed, etc) in buckets to flush the toilet - just pour a gallon or two into the bowl and away it goes.
A wind-up radio is a great idea, as are some flashlights and lanterns. I buy batteies in bulk at Costco and store them in the freezer so they will last longer, just let them warm up before you install them in anything. A battery powered or wind up clock, books, board games, etc are good ideas as well.
Other things to do: Get some 2 liter soda bottles, and fill them most of the way (not all) with water - then freeze them. Leave them in the freezer and if power goes out, they will help keep things frozen longer. Get some first aid training, and then make a first aid kit (the commercial ones are usually junk). If you need any special medications then get an extra months (or more) supply. Stock pet foods.
BTW, all of these preparations are good for all sorts of things, not just a natural or man made disaster.
Good luck!