Well the best things are supplements from a fit shop or something.In your case you could buy yourself a bottle, where you will have your water in, i had been in your place when i was your age, now i drink 1l in the morning right before school.Just to stay hydrated and your urine must be as water not yellow, than you know your body is cleaned.For the diet you can find one on bodybuilding's website.
To gain weight, you simply need to consume more calories than you are burning.
I would recommend high carb foods like pasta, rice and bread etc. Protein shakes will help add mass too.
I am like you, you have to eat alot like Pasta and meat. Also buy protein and muscle milk to help you gain extra calories.
Start eating alot of protein, and forget all the crap, candy, chips, cookies, pizza, soda, energy drinks.
ALL CRAP
Then eat more vegetables, fruits, wheats, proteins
And you need to start eating more,
maybe hit the gym 3 days a week for starters too