Though this question's not for me, I'll answer it anyway. Metabolism is your body's use of energy and ultimately, you want your metabolism to stay high, but that doesn't mean you can't gain weight. To add weight, you want to increase your food intake and do strength training exercises, maybe some heavy-weight lifting.
You want to give yourself a good challenge so you're getting stronger and adding some bulk. For weight gaining foods, I like steak or roast beef and chicken is a superior source of protein.
Now if you're set on slowing down your metabolism, I'll say eat 2 big meals a day, nothing in-between. By doing this your metabolism will drop, but if you're anything like me, you won't be as hungry. Ultimately this method will give you more fat and less muscle, and you may start to feel lethargic.