How does this diet sound? i dont think i eat enough calories?

ok so... I have a small bowl of musli for breakfast then a tuna and cuber sandwich/plain bread for luch then a fruit yoghurt for a mi-afternoon snack/banana or other fruit then something for tea ( I cant control that because my mom cooks it). for exercise, i sometimes run but i get out of breath really easily so it isnt particurlaly useful. there are no clubs around but i walk my friends dog most days through fields and run a little with him. I counted my calorie intake and it seemed i didnt get enough. Is this a bad thing for losing weight? I'm sorry i'm ill educated on the topic :) also can you think of any other exercises i can do for free in my own home or surrounding areas? (i like somwhere quite open) i am aiming to lose around 3-4 stone/ 3 dress sizes so is this a good diet or not? I'm too young to get into the gym (15) so what else could i do. I'm not big on exercise but i used to and am more than willing to get back into it. any sugestions? and how long do you think it will take to lose the weight at this rate? thank. ps. sorry for my bad spelling i was tired when i wrote this lol.