You will need a scale and a book that has nutritional information on foods. It also shouldn't be too difficult to find this information online. For something like an apple, always count it as 80 calories. If you have a slice of whole wheat bread, measure how many ounces you are having. Whole wheat bread is about 70 calories an ounce. It gets more complicated when you have something like a sandwich with lots of stuff in it. Then you will have to estimate a lot. In this case, I usally try to compare it with something similar in my book. The book I use is called Food Counts.

Go to the book store and buy a nutrional guide.