Consume things that have no calories. There are two things you eat and drink that contain no calories: water and fiber. The more of these you get into your diet, the better off you will be. For example, you can eat a pound of mixed salad greens with assorted raw vegetables (carrots, red cabbage, celery, broccoli, onion, etc.) with a low or no-calorie salad dressing and only have eaten 100-150 calories. This is because of the high water and fiber content of the salad and low calorie dressing. Also, eat lots of celery. It has only 8 calories, but amazingly, it takes more than 8 calories to digest it. So, you actually BURN calories by eating celery! It's not much, about 2 calories per 8 inch stalk, but it sure doesn't hurt.
Avoid sodas as much as possible. Instead, drink flavored water or unsweetened iced tea.
Take broth-based soups. They are a relatively low in calories. A commerically-available ready to eat "chicken and stars" soup may only have 80 calories for 1 serving, much less than diet "meal shakes" or nutrition bars.