Re: How can I gain some lean muscle?
Eat a lot of protein, eat a lot of carbs, and eat a lot of fat. Try to gain around 1-1.5lb a week. To give you an idea of how much to eat, for myself (182lb, 6', male) I eat 165-180g of protein, ~300g of carbs, and 80-90g of fat.
For exercise, I suggest a program like Starting Strength. You can find the book online, but basically it makes you do squats, deadlifts, overhead presses, chest presses, barbell rows, and dips.
If you plan on weight training at your age, make sure you eat A LOT or else it can stunt your growth.