Diet

Don't you just hate this word? What does the first three words spell? DIE! You can go on a high fat diet and lose weight or go on a all veggie diet and gain weight. Don't believe the fad diets. Just consume clean foods at moderations and EXERCISE! If you eat more than you burn, what happens? If you eat less than you burn, what happens? If you keep these two things in mind then you won't have any problems putting weight on or putting weight off. Just keep in mind that it is best to keep as much muscle as possible while trying to lose weight. It is also best to lose fat weight instead of muscle weight. The more muscle you keep, the more leaner you'll look and be. If you lose muscle, you end up looking soft and actually feeling soft. When you diet, use these numbers to guide you with your diet. (40c/50p/20f) So, if you're trying to consume a 2000 calorie diet, your macros will look like this.
Carbs = 2000*.40/4 = 200g
Protein= 2000*50/4 = 250g
Fat = 2000*20/9 = 44.4g

Or you can do it this way. If you way 200lbs with a body fat% of 15, your lean weight is 170lbs. Again, this is based on a 2000 calorie diet.
Carbs - 170*1.5 = 255g /1020 calories
Pro - 170*1 = 170g /680 calories
Fat - 33.3g / 299.7 calories