Lose 40 lbs in 5 months??? BIG DEAL?

okay im 15 5'2" and 160 lbs unfortunately , i want to be 120 lbs by august just because my 16th bday is then and i start school soon after..so i just wanted tips on how to get there and if it is even possible i have access to my local gym and semi healthy foods. but i cld try to get my parents to buy healthier, and i would also like to know about what size i would wear if i were 5'2" 120lbs thanks