I know how terrible this might be to you. My uncle had the same problem. Fast food is really delicious and it is difficult to resist the temptation. Let me give you some advice to help you.
First, you should try to eat healthier foods. Why don’t you try to eat some salad with your sandwich. If you like fast food so much, you could consider starting to have smaller portions of junk food with some healthy food like broccoli, for example.
Another thing you could do would be start exercising. Why don’t you take up swimming, for instance? They say swimming is one of the most complete sports. Also, you should try to walk more. You could start walking to the supermarket or to school, for example.
Finally, if I were you, I would try to include some fruit on my diet. If you eat more fruit, you will feel less necessity of eating things like chocolate or drinking soft drink, for instance.