Outside of all the criticism you have received for poor exercise, poor diet, and AAS at a young age (all of which I agree with), I did notice this portion of your post. I ask that you clarify what your testosterone levels are, assuming you got them tested at the doctor's office. If you could post the specific number and scale of measurement we could possibly give you better advice.
If you meant to type "100 ng/dl," you have very, very low testosterone. the normal range for the ng/dl scale is 262-900 or so, and being 16 you should be towards the upper half of this range. If you are below 262 you have an endocrinological problem and are not producing enough testosterone naturally, which could explain your very low weight, and would also explain why the weight that you are gaining is mostly fat.
If you do have low testosterone levels you will need to receive testosterone replacement therapy. This will bring your testosterone levels into the normal range, but certainly is nothing like a bodybuilding AAS cycle. At your age you do not need, nor would it be more productive, to use AAS in a bodybuilding sense. Check what your numbers are and let us know what they are.