How do you lose weight by just working out?

Two things:

One - You can't lose fat in just one specific area, and you can't target that area. When you burn fat, it burns it evenly throughout your whole body.

Two - You CANNOT lose fat if you are not in a caloric deficit. That is, eating less calories than you burn. The more calories you burn, the more fat you burn, but if you eat those calories, the fat comes right back.

The best fat burning workouts are cardio workouts over long periods of time. Examples of good cardio are swimming, running, walking, jogging, dancing, cycling, and stair climbing to name a few. Good luck.