How to stop eating junk food?

I cant stop eating junk food!! No matter how hard I try I just cant! :-( I gained maybe 20 pounds, idk I gained weight. Im worried that my dress wont fit me for my cousins quince(im one of her damas and its in two months) I tried diets but I end up eating junk food!! I need to loose 5-15 pounds before july 23 plzzz help!! Any advice or opinions would help