Well the first thing to take care of when trying to gain weight is to make sure you’re in a caloric surplus.  As someone who works with a lot of high school athletes, I’ve found most teenage athletes do not eat enough to gain weight.  ”But Coach, I eat all the time!” is a very common phrase.  When it comes down to it, they’re not eating enough or they’d be gaining.

Now, once you’ve determined how much food it takes to simply maintain - basically your weight week over week is relatively the same (your daily weight will fluctuate based on fluid and food intake) - then it’s time to start upping the calories.  Start small and up it by 500 calories per day.  So if you were needing 3500 calories to maintain your weight, you’ll now eat 4000 calories per day.  Do this for 2 weeks.  Still no change in the scale?  Up it again by 500 calories and do this for another 2 weeks.  Find a caloric level that allows you to gain weight at a steady and constant pace - around 1-2 lbs per week.  

Nutrient choices - to mitigate fat gain, get your calories from quality sources.  If it poops or grows, it is usually a good source of food.  This means animal products, veggies, fruits, nuts, etc.  

Nutrient timing - I’ve found the biggest thing for me and many of my clients, is keeping as much of my calories as I can around my workout.  I take in approximately 70% of my calories in a day around my workout (hour before, during training, and the hour to two hours after training).  Doing so gives the body calories when you actually need them.  So going back to my 4000 calories a day example, that means you’d take in 2800 of the 4000 calories in that hour before, during training, and hour or two after training.  As said this has been the best help for me keeping bodyfat in check while gaining weight.  Some say don’t overload the calories like this but in all my clients it has been beneficial and research shows that it also is very beneficial.  Logically it makes sense - use calories when you NEED them.  From a sciency standpoint your training increases the MTOR and Glut4 translocation responses allowing for maximal nutrient uptake in muscle fiber in conjunction with a very high insulin response.  Insulin being the most anabolic hormone in our body, is best used after you just placed a huge demand on your muscles signaling your body to repair muscle/grow muscle instead of storing nutrients as fat.

Lastly, but definitely not the least important, is training.  Lift heavy stuff.  Require your body to grow.  If you’re doing endless reps, and tons of cardio you’re really not giving your body a reason to grow.  Be a viking, lift heavy stuff.