How much weight will I lose with this diet?

Hi, i'm 15 and keeping my height, weight, the number of days I exercise per week in view, I need about 800 calories a day. I have a Kellogs cereal for breakfast and lunch. One bowl has 113 calories which means by the time I've had lunch, I've had 226 calories. I have a digestive biscuit or a fruit if I feel hungry before dinner. A digestive biscuit has 74 calories. By dinner time, I've had 300 calories. I have a breast piece of barbecue for dinner which I'm not sure but has around 257 calories I think. Which means I have 557 calories a day. I eat more digestive biscuits if I feel I'm taking very less calories. I want to know how much weight and in how much time will I lose if I continue this diet. Thanks. :)