How long will it take me to lose 3stone?

I am 5'8 and I am 13stone I really hate how I look although I don't think I appear to look as if I weigh as much as that but still anyway I really want to lose 3stone and I know it's not going to be quick. I joined the gym a month ago and I have started going around 4-5 times a week and burning around 300-500 calories there! At the moment I haven't thought of a diet plan but I have cut out much of the unhealthy food I ate before! I'm thinking of eating porridge for breakfast or something like egg! Then for lunch tuna salad and fruit and for dinner something like a soup or chicken and salad? If I was to eat around 1,200 calories a day and burn around 300-500 calories at the gym how long would it take to lose weight? Ps I am 17 (I'm not sure if that makes it easier to lose weight being younger?) But yeah thanks :-)