How to not go into starvation mode?

Okay well i need to lose a large amount of weight as a i want to join the Army. Im 16 soon to be 17 years old. Im 6'1'' and 295 lbs. I want to be able to drop atleast 40-50 lbs. anyway, Ive been looking around calorie calculators and seeing at how much calories i burn at rest and how much i burn while exercising. First off, let me tell you my movements. MWF, i do a video tape that walks me 1 mile. Then i go to work and walk there which is 1.3 miles. I walk around cleaning a building from 5PM - 6:30PM (not sure if its a mile atleast although im constantly moving) then id get picked up in the winter. When summer comes ill walk back which is another 1.3 miles. on Tuesdays and Thursdays, i have no work so i do 3 miles on the video tape which is brisk to intense walking. How many calories should i be consuming so i can lose weight? Im really into salads and spinach and eat them all the time as a filler. Also love tuna. I regularly eat 2 cans a day with the salad. Please help?