Right fella you want to gain size.
This is without supplements (as yet) and would consist of you breaking up when you eat throughout the day.
Most of us eat breakfast/lunch/dinner... and possibly a snack for supper whilst watching the game/film...
You would need to start say eating a breakfast meal, then an hour/two hours later eating something light then for your lunch eating something, a few houres after this eating something, then eating your dinner, then a few hours later... you get the idea, so overall your having around six meals a day...
these meals dont necceserally have to be large meals, its much better for our human body to actually eat lower amounts of food throughout the day rather than three large meals, as it gives our body chance to break down the food parcels and beenfit from the nutrients, rather than taking on loads of food , getting digestive problems, then breaking it down.
This something your supposed to eat... well you need Carbs to build up the size, and protein to develop muscle and definition of the muscle. there are carbs in your big mac meal, and there are carbs in your tuna/pasta dish... you guessed right, the big mac meal is full of empyty carbs thats just gonna add fat.
Positive carbs (pasta), proteins (fish, chicken) are the more common choice, and are inexpensive.
fish has to be your new best friend, i mean you gotta start swimming with the fish.
Me personally i dont eat six meals a day as im not really bothered about my size, im more bothered about my strength goals, and i used to weigh 14st (like 196lbs or something) now i weigh around 70kg (154lbs) and i can actually lift far heavier weights than i did then... So size isn't everything kid.. you may see bigger guys in your school or whatever and think: damn they must be strong or whatever, but you stick to a healthy diet of pro carbs and proteins and even if you stick to your weight, if your weightlifting and increasing the weight you lift... the bigger guys who dont eat right and hardly train would get walked all over in the gym by you.
Check out bodybuilding forums, i cant ive you a link on here as i dont think its allowed, but google search for places like bodybuilding dungeon, here theres bucketloads of actual diets for free than you can go by for your goals.