Having a healthy metabolism is good ... it means your body will use up what you put into it.
Questions are ...
What goes in?
And what does your body use it for?
Do you do resistance exercises? (weights, and that sort of thing)
If you do, eat your normal diet, but on days that you're working out, eat plenty of carbs (pasta, banana, avodados, etc). Afterwards, your body will try to repair the damage you have done to it ... so FEED your body with protein ... eggs, skimmed milk, fish, red meat, chicken.
That will build good muscle tissue, which is the only sort of weight you want to be putting on.