I've been studying exercise science and nutrition for a number of years now. I do this because I think it's important. I've noticed that Americans put a great deal of importance and study time into skills that increase productivity, but few ever study the information most important for life: how to eat. In my experience, I've found that there are 5 inexpensive tools that are really important to healthy eating. I've also found that most homes rarely have any of these things.
1. A digital diet scale: essential to knowing how much you eat and how much you should eat for different goals. I've found that when I try to lose weight, I always eyeball too much, and when I try to gain weight, I always eyeball too little. A scale allows me to ensure that I eat enough for my loss or gaining goal, and no more. Available at most home stores, groceries, and department stores - $10-$50
2. Measuring cups and spoons: essential for the same reasons in #1. Many reference materials will give you serving sizes by weight, but they may also give them by volume. I like to have a few extra cups and spoons around so I'm not constantly washing them. Available at most discount stores - $2-$5
3. Macronutrient reference: essential for knowing what's in a food when the package doesn't tell you. If you eat mostly fresh foods (the ideal dietary goal), you will have to know the values of macronutrients in each. Everyone should have a way of finding out how much protein, fat and carbohydrate are in most foods. Available free online or at most book stores - $6-$20
4. Gylcemic Index reference: essential for controlling weight and preventing diabetes. Everyone should try to compose their diet from foods with a generally low GI. Controlling blood insulin spikes is a life or death matter. Available at most book stores - ~$4
5. General Nutrition reference: essential to learning and troubleshooting your way through our over-manufactured food market. Having a book with sound dietary principles will help everyone to learn the basis and reinforce them over time. Available at most book stores - $15-$80