You are still growing, so you can afford to eat as much as you want and not worry about it. It takes protein to grow muscle, so eat plenty of protein - but a somewhat balanced diet is always a good idea. There is nothing you can do about your wrist area - it sounds like you have what I and countless other people have, which is a high forearm muscle. There is no way to pull it down to your wrist - you can make it bigger where it is but mine for instance only come about halfway down from my elbow to my wrist. There is no substitute for a gym when it comes to weightlifting. There is a lot of trial and error involved when you are first starting out. It took me 1-2 years before I saw results but then they came fast and furious. Stick to "basic" movements that involve several muscle groups at once i.e. bench press, shoulder press, chinups, leg presses etc as opposed to "isolation" movements that involve fewer muscles and less poundage (i.e. curls, leg extensions etc. Ultimately I found that it is very easy to "overtrain" and do too many sets. Your body doesn't grow because it is too overwhelming and to taxing on it. Doing less sets but putting more into to each set and going all out got more results for me. Again, a lot of this will be trial and error. The key is sticking with it, through the good times and the bad times. If having a killer body was easy, everyone would have one. But if you are determined, there is no reason why you can't bulk up enough to get rid of insecurities. Remember, if you want to put on weight then you must eat more. Your body is not putting on weight with your present diet so increase the amount you eat (hopefully good foods, not junk and fast foods). Good luck.

You're only 15!!! The first thing you have to do is eat ALL THE TIME. Drink milk between meals. Take some protein powder if you're not building any muscle.
The second thing is to go to bed before 10:30 pm. If you don't go to bed early your body won't produce growth hormone, which it only does in deep sleep, quite early in the night.
Third, you have to keep your stress levels down.
1. EAT
2. SLEEP early and often.
3. BE HAPPY!
If you still feel too skinny when you're 18, then you can start to worry a little (not too much)
Also, you could take small amounts of Bovine collostrum, which will nourish your endocrine system.
The best thing you could do is learn about good nutrition. learn how to cook whole foods and enjoy them.

Keep us posted!