He mentioned healthy gaining
OP - make sure you eat a lot of protein, at least 1 gm/pound of body weight. Eat slow GI carbs like oatmeal, brown rice - you *can* have faster GI carbs but reserve those for your post workout shake and the meal after that.
Lift heavy for 5-6 reps max - but not so heavy that you injure yourself. You should know what weight you can handle for a clean 6 reps. (6 is an average number, some people get results with 3, 7, whatever - just make sure you're getting an insane workout)
Post workout nutrition is very important - chug a 40-50 gm protein shake with 30-40 gms of dextrose - great for spiking insulin and shuttling all the nutrients into your muscles.
And no, don't ever have mcdonald's and ice cream if you want to gain weight (unless all the weight is on your stomach and your butt)
Edit - forgot to mention calories. You're 15 so you have a fast metabolism and you play sports - so you're going to burn everything pretty quick. You need to ingest 3,000-3,500 calories. Make sure those calories come from good wholesome foods and protein shakes and you'll pack on the pounds slowly.
It's very easy to ingest 3,000 calories in a single fast food meal - but quite hard to do when you're eating chicken breast with brown rice, or 8 oz of lean meat with veggies. It gets real boring!
WOAH! Great info for the most part ... but we ALWAYS start with proper nutrition.
1. Start planning your meals: 6 per day.
- This can include 4 meals (1 a healthy snack) and 2 protein shakes.
- Try to schedule your meals 1.5-2hrs apart maximum.
7:30, 10 (Shake), 11:30/12PM, 2PM (shake), 4PM (shake/snack depending on practice), 7PM (proper Dinner!), 9PM (shake)
- Prepare your shakes (powder) in a bottle (preferably Stainless steel), don't reuse the Gatorade bottles more than twice.
- Meals don't have to be HUGE just portion size.
- Oh and do NOT start out with egg yolks (there is a REASON why Scharchenegger had a heart-attack)! Eat Tuna/Fix (great protein, calories & Potassium/Calcium ratio) plus you help get the Omega fatty acids (6+9 I think). Flax seeds is 3 I think.
If you begin right away to consume 3500 Calories a day [old world thinking of 1.5-2x the calories for your weight) you're going to either have the RUNSS & ***** yourself during your workout or games OR you'll feel like someone parked a TANK in your gut for 3 hours!!
2. write down what you eat & also your workout schedules!!
You NEED to do this. Just like any major financial planner studies your goals and saving/investment habits the SAME must be done for your eating & workouts.
- this is MOST important because it will show you on the days your Strongest, when you have the MOST energy, and when you're plateau'ing on your workouts.
It IS possible to get 30lbs in 90days easily; I've personally done this before.
3. Never forget your fruits and assortments of vegetables this is USUALLY ignored when supplements are taken.
4. When you begin your workout think of #2. After each exercise write down what Set & Rep you begin to feel a) difficulty, b) easy, and c) which arm was weaker (dumbbell curls).
Now onto your workouts.
You need to FULLY READ the proper motion of your exercises and pay CLOSE attention to your body. Not EVERYBODY will get a huge chest by Barbell Curls alone - for me with my shoulder injury this never worked. I couldn't bench press over 145lbs if my life depended on it (when I was 145Lbs or 189lbs now)! However I can easily Dumbbell Bench press 60-80lbs per hand (50-75Inclined) at 4x10 reps when I used to work out: it'll take me 2mths to get there again.
1. Concentrate on Compound muscle group exercises.
* Squats - Barbell (not hack squat machines)
- eventually you can use a plate under your toes for more impact.
^ VERY important as your blood is made 70% in your themur bones.
* Barbell Curls (pronated & supinated - equal reps) - you may not be able to write your name if you really push it but its GOOD.
* Bent Over Rows - Barbell. This will hurt your back unless you bend at the knees and hips. Alternative is to use the Hammer Strength Smith machines at the gym (there are 3 and they do wonders). Or Dumbbells keep back straight.
* Bench Press - Barbell.
* Inclined Bench Press / Shoulder press with dumbbells with palms facing to your chest (KEEP BACK STRAIGHT not curved into your stomach)!
* Dumbell Flys - chest again keeps the tendons strong at the connection between the muscles under your chest.
Now also don't forget the naturals.
Chin-ups. (Pronated & Supinated) You'll be amazed how many kids your age cannot do 2sets x 10 reps of their own weight properly (bending our elbows not your wrists).
Push-ups. I like the mod I did when my boys and i where your age.
When bored round up the guys (20+). First guy does 1 push up. Second does 2, etc etc. EVERYBODY goes until they fail! It'll feel like the hand of the devil pulled your chest to smack your face in the ground but it works a great pump! If #16 fails. the NEXT guy does his 16!
Everyone rests for 10 minutes. Start again at 1.

Do this with chinups too.
If you can do 6 Sets x 10 reps of Chinups of your own weight resting LESS than 40-60seconds (no more than 1.5minutes) then start using weights between your legs (25lbs and start 2 Sets of 6)
Also if your school has parachuts to use for wind sprints use it.
Protein shakes. Get single packets first to see the taste and quality when mixed. With Water or with 2% Milk at your age is best.
Cold Filtered Whey Protein Isolate and the primary source is THE BEST!
Casien (milk products based) is great when you cannot sleep and have a shake with lots of this and go back to sleep (long term absorbtion). Remember how mom used to give you warm milk to help you sleep ? Trypophan

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i'm sure there is a LOT more advice but remember to eat healthy and lean. you want your muscles to get bigger, not the layer of fat under the skin. You're not going to Alaska or Arctic Circle for 4mths in Winter are you??
Cheers and good luck.
Keep this thread alive - with pictures of your gains (I don't want your american pie full body shots lol).
Keep practicing as well!!