Start with the basics: food, water, energy [lights, heat, cooking]. What do you eat? Stock up on it. Buy an extra can or box every time you go grocery shopping. Or two, if it's on sale. Or three if you're feeling flush. Buy some bottled water [one of the only reasons to purchase the stuff unless your tap water is crap].
Then think of how you would do the ordinary things every day without power. How would you heat water? Wash dishes? Take a shower? How would you cook?
Look around at what you already have, like camping gear and the stuff in your kitchen. Got a can opener? Got a pan or two? You're already stocked up on some stuff; time to fill in the blanks.