Teen Weight Loss Help!?

im 15, 149 lbs & 5'3. Im not big but I could ne skinnier. I was wondering if anyone had some suggestions for me. its hard ti have a balanced "diet" because whatever the parents chose to have that night for dinner is what we eat. & I have been doing small portions i've also been active with exercising. of you could tell me what I can do to lose as much weight as possible starting today until july 31st. were it will actually make a noticeable difference please tell me.