When I was your age and in high school, I weighed about the same as you. I was one of the smallest girls (weight) and I used to play a lot of sport - hockey club, athletics club etc. I'am 21 now and weigh around 9 stone.
You don't want to start eating loads of pizza's etc to put on weigth 'cos you'll put it on your stomach and this can be unhealthy (that would be my advise if you were a bit older) however, with you only being 14 it's a difficult situation because with RDA of fats etc they differ from child to adult which I'am sure you'll know. Your a growing boy and you need vital foods for growing up.
I'am currently studying Nutritional Health and all I would do if your desperate to put on weight would be to enjoy your food. There's ways for people to put on weight healthily by logging a diary of how many calories/saturated fat etc you have in a day. However, things like this can lead to people being obsessed with their weight.
All I can say is that I have always been a picky eater and in my later years of high chool then college I put more weight on even though I ate very little I did a lot of exercise. You need to make sure that the energy that you are using for sports etc are equal to the amount of energy that you are intaking from foods. You don't want to be using more energy than the amount you are taking in as this is a way to loose weight.
Idealy (spelling) if your worried about your weight I would seek medical help as they can always put you onto a Nutritionist.
Hope it all goes well and hope this may have been a bit of help for you.
