You might have a fast metabolism but it would be very useful if you shared an example of your daily intake so we can see how much and what you eat.
You should eat a lot of proteins and the right ammount of carbs and have enough sleep (at least 8 hours a day) in order to gain quality weight.
When I started to train I went to the gym 4 times a week, each time exercising 2 muscle groups and abs and used the following diet:
3 egg whites
1 cup of cereals with low-fat milk
sanwich with integral bread and turkey ham
pasta or rice
300g of meat (turkey, chicken, horse or fish)
300g of meat
If you are a healthy person, eat the right ammount of quality food, exercise well and sleep enough sooner or later you will get the right results. Many people complain about not being able to gain weight but in most cases it is not because of metabolism or health issues but the matter of incorrect daily habits.
Hope it helps!