What is a good way to lose weight with leg work and sit-ups?

Not quite right"?" Really it is not about losing weight per se. Ideal situations increase leaness by: 1. Losing fat. 2. Gaining muscle. Exercise can do both. If you are firm and lean, you will look better and be healthier.
Weight Loss. This is a very simple issue. Weight depends on the balance between intake (how many calories you take in), versus output (how many calories you use up). In order to correct the inequities involved in your becoming overweight you must reduce the number of calories or amount of food you eat, and increase your utilization of these calories by increasing activity (exercise, etc.). Both take effort.