This is very complex. It's a blend of nutrition and exercise. Please understand that your "health" is more important than "bulking up."
As someone who struggled with weight gain, I understand that gaining weight can be as difficult as losing weight, and it's not about eating ice cream and cookies.
You want to build healthy muscles. Your heart is the most important muscle in your body. The proteins that enhance heart health are the best for the biceps and triceps too. Do your research. Google heart healthy proteins to see why Omega-3's in fish & nuts and lean meats matter. Learn what carbs, such as whole grains, give true boosts without triggering an insulin crash. Learn about the essential amino acids that can't be made by the body and must come from food. Learn why resting between workouts is so important.
In other words, don't give yourself a goal for the mirror to reflect. Empower your mind and body to be healthy. This true inner strength will reflect in your life and the mirror.