I very recently joined a gym to gain muscle, not to lose weight. I've been going there for about 4 days and I've gained a few pounds of muscle. I'm 15 years old and want to know what I should do to gain muscle weight eventhough I have a fast metabolism. I'm trying to eat even more than I normally do but I'm not sure if it's enough. Please give me some advice.

Nutrition is key. If you wanna gain muscle, consume at least 1 gram of protein per pound of bodyweight, but 1.5g is better. Lean protein such as chicken breast, turkey, lean beef, fish and seafood, egg, and low-fat cottage cheese. You can eat at least twice as much carbs as protein every day but your carbs should be brown rice, potato, oatmeal, and vegetables instead of processed and refined products (e.g., sugar). Consume healthy fats (flaxseed, olive oil, fish) with your salads or other meals. Eat 6-8 times a day. Have a whey protein shake with dextrose tablets post-workout. Also you can take creatine and glutamine as supplements.
Training 4 times a week is good, train each muscle group only once a week to avoid being overtrained. Limit cardio to once per week.

Thank you very much! I'm gonna start following your advice.