At 19 years old weighing a whopping 112 pounds you could build muscle just by eating a lot (not even eating well) and doing just about any physical activity.
There is no reason you should even consider gear right now; unless you are completly missing your testicles.
As an easy tip to get you started, force yourself to eat 2 peanut butter sandwiches after you eat lunch and 2 more just before you go to bed.
Your body just needs calories.
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