What can i do to slow down my metabolism? it is too fast and i can't gain any weight?

Depends: If weight is normal and you can't gain weight it may be less of a problem than if your are underweight and malnourished. Starting with your doctor to rule out metabolic causes such as hyperthyroidism is very important, then based on results consuting a licensed nutritionist may help.

Eat and exercise.: You need to focus your exercise regimen more on weight training than aerobics. As well, make sure you eats lots of food, and focus on complex carbohydrates and lean protein sources. We don't focus on slowing down a fast metabolism; we work with it.