It's really tough to give a 13 year old advice on this kind of thing, and I'm by no means a qualified dietician, so I'm not even allowed to try to give you really specific advice. One reason that it's difficult to give advice is that you are still growing, so you still need to take in adequate nutrients to avoid development issues. If you try to restrict your calories a lot, you can end up with a whole bunch of other health issues, either now or in the future.
Probably some of the best things that you can do in terms of changing your nutrition for the better is to remove things like soda, candy, snack foods, fast foods, etc. from your diet. You can have these things once in a while, but try to avoid them most days and when you do have them, have them in moderation, don't go overboard. Also, make sure you eat at least 3 meals per day, somewhat evenly spaced (breakfast, lunch, dinner). If you have long spaces between meals, have a snack, but try to plan that snack ahead of time to make it healthy, rather than just grabbing anything that you can find. Have only one plate full at your main meals - don't go for seconds, thirds, etc. Try to fill your plate with more veggies than anything else (think 1/2 your plate of at least 2 kinds of veggies or veggies and fruit).
What country do you live in? Your goverment's health website should have information on recommended healthy eating habits for kids your age.
(If it makes you feel better, I wasn't overweight at 13, but I had (well, still have) stretch marks - no idea why! It really sucked, but couldn't do anything about it.)