First you must be quite lean, then you must eat only clean food.
For maintenance eat about 18 times you bodyweight in pounds (200 pounds would be 3600 K/cal a day).
For gaining weight, think of a target body weight. Add 15% of that to the total and multiply that by 25. Thats a basic idea of how much you should be eating.
Look around the site and you'll find much more info...