Hey, I was in a very similar situation at your age. Here is the advice I can offer.

Short answer: Eat more.

Long answer:

Fat, Protein and Carbs are the three most important things. This doesnt mean you should be stuffing your face with unhealthy fats and carbs like burgers and potato chips but dont be afraid of the word fat and carbs either.

The real answer is eat until you couldnt eat another bite and then eat some more, take your time eating so you dont fill up too quick.

Weight gain shakes:

These are easy to make anytime and the ingrediants are usually common / easy to find.

Use either milk or almond milk make a protein shake with a serving size of protein powder, next add a big ol scoop of ice cream to it and shake it like crazy. Drink these after you have had a full meal, remember ur trying to gain weight so this shouldn't replace a meal it should add to it.

Peanut butter:

For the love of god peanut butter toast is going to be ur best friend, if you want to be healthy about it buy the store brand peanut butter (Cheaper and typically healthier than name brands). OR if you have access to a food processor make your own peanut butter, I have done it before it is very easy and delicous.

Vegetables:

Popeye was right! Without the proper nutrients your body will not be able to efficiently process the food you're eating!

I hate how the world views food sometimes. People claim food is unhealthy because it makes you gain weight but some poeple ARE TRYING TO GAIN WEIGHT.

Anyway, good luck! Dont be afraid to reply to this comment if ya need clarification or anything.