I need some advice about dieting?

im about to graduate from high school to make a long story short i need a diet i dont feel comfortable and i cant buy diet foods because i have no money and i want to loose as much weight as i can by the time i graduate in june and i have a project that i have to do to graduate so im occupied with that on the weekends i dont know what to do…
PS im not totally helpless i was in the marching band for four years but im getting bigger and they started marching band again but i can go to practice because whoo hoo SENIOR STATUS<3