Look at what you have in your fridge, cupboards and freezer. What meat do you have? What grains? Vegetables? Fruits? What staples do you need?
I spend the least at the store when I am completely organized and plan every meal of the day. Breakfast, lunch, dinner and snacks. I look at what I already have, what we like to eat, and what kinds of dishes I have the most ingredients to make. For example, if I have ground beef and tomato sauce I can make spaghetti and meatballs or chili and all I need to buy at the store is pasta or beans. I always try to make meals that have a meat, vegetable and a starch. We usually eat fruit for snacks. I also try to make certain that I have fresh fruit for the kids to eat and frozen fruit in case we run out of fresh before I can get groceries again. I only shop every two weeks and try not to go to the store between trips or I end up spending too much.
You can find an example of my menu here.