You are still growing, but that doesn't mean you can't start building a linebacker body by the time you are finished with natural body development.
You should definitely start a proper diet:
5 to 6 small meals with 60% carbohydrates, 30% protein, 10% fat, each day.
6 oz. chicken breast
2 cups cooked rice
1 small apple
2 tablespoons peanut butter
For weight training, work each body part once per week
Use moderately heavy weight
Day one: LEGS
Leg press: 3x10
Leg curls: 3x10
Standing calf raise: 3x10
Day two: CHEST
Bench press: 3x8
Incline Bench press: 3x10
Incline dumbbell flies: 3x8
Day three: BACK
Wide grip pull downs with cable: 3x8
Close grip pull downs with cable: 3x8
Seated rows, close grip: 3x10
Day four: SHOULDERS
Seated dumbbell press: 3x8
Standing side lateral raises: 3x8
Rear lateral raises: 3x8
Day five: ARMS
Dips: 3 to failure
Standing tricep press downs: 3x10
Standing barbell curls: 3x10
Seated dumbbell curls: 3x10
Note: Go on youtube and look up each exercise I have mentioned to see how it is performed if you are unsure of what each one is.
I promise you will see results in about a month if you STICK WITH IT!
Believe me, people will notice it before you do.
Good luck, I was in your shoes when I was your age. I thought I would never gain weight. Boy, was I wrong!
25 years Powerlifting experience