Your car won't run well if you don't fill it with the correct fuel. The same goes for your body especially before a workout. You should avoid foods that are high in protein and fat.
It's important to eat foods that contain carbohydrate for energy and a little protein to keep up your energy levels. So, if you're working out in the morning, try eating a banana, some cereal, or oatmeal.
If you workout mid-day, try a turkey sandwich on whole wheat bread or a piece of fruit.
But if you save your workout for after work, eat a snack before, then eat dinner after you're done exercising.