I was your age when I started saving up. I bought bulk amounts of Pepsi cans and Mars Bars, then sold them at 200-250% of the price I bought them for to other people in my year at school. To put it in a simpler way, each Mars Bar cost me 20p, and I sold each one for 50p, whilst buying the cans for around 18-20p and also selling them for 50p each. I was making £60 a week profit, and selling around 100 mars bars and 90 cans per week. It was a pretty amazing time of my school career, and as a result, I was able to afford my Macbook.