Honestly it sounds like you already have a good idea on where you need to work -
Cut out the junk. I adore chocolate as well as much as the next woman, but all those calories and saturated fats hurt your diet so bad. If you are like my mother- who will eat an entire tub of ice cream in one night and cannot say no to chocolate- you may have to remove it from your diet completely. She doesn't have the will power to leave it to a little. If you're not like her- substitute one large bar of chocolate with a small piece of very dark chocolate. These are still high in fat and calories but you will eat less and therefore consume less. But cravings are usually pretty satisfied by them.
Unfortunately chocolate and sweets aren't the only kind of junk out there. Most processed foods have tons of sugar, fats, and calories without enough nutrients. In my opinion these processed foods include the obvious fried foods, fast foods/take away (anything in a box), and things like chips and soda (including diet). But they also include white bread (in the US I know they add high fructose corn syrup to it), lots of different pastas, packaged dinners (hamburger helper), canned or processed meats (meaning SPAM over tuna), and many sauces and dressings. So for example Barbecue sauces and marinades can have tons of sugar and sodium without anything good in them; salad dressing can have TONS of fat in them and make the meal worse than a hamburger.
Try to choose wisely-avoid take out, classic "junk" foods, and prepackaged meals and sauces. Aim for foods that are whole grain, lean meat (chicken breast over a large rump steak), low fat dairy, and high in steamed veggies. (Canned veggies kill a lot of nutrients and add lots of sodium.)
So what you need to do is first- talk to your family. It takes a lot of support to do something like changing your diet- especially if you aren't the one buying the food. Ask them to commit with you a better, healthier lifestyle by not buying or storing junk in the house. If they wont do this with you you will likely have to cook and prepare your own foods.
Then make sure you eat every meal plus a snack or two. This is extremely important. If your body feels stressed or that it's not getting enough food it goes into starvation mode and just stores everything. Start with a good breakfast- whole grains. Avoid the sugary cereals, the idea of a large stack of pancakes, or lots of fried eggs and bacon. Aim for a breakfast with some protein and fat- nuts, dairy, egg, etc. AND gains. Personally I go for oatmeal with soy milk, blueberries and almond meal. Lots of fiber, good fats, protein and enough fuel to start my day.
For lunch, because you're a teen and therefore you have school - it might be a good idea to bring your own. I know there has been a lot of changes since I graduated, but if they're still serving hamburgers, chicken burgers, pizza, and occasional salad containing only iceburg lettuce -that's no way to go. After you get home try to have a snack such as an apple, yogurt, boiled egg, etc.
Dinner will be the tricky one. If your family eats out a lot you have to learn how to order. Avoid sauces like mayo and salad dressing and maybe add them on yourself. This controls the portion added. Also go for grilled over fried. Beware of the idea that salad is always healthier. I used to work at a fast food joint and the Cesare salad we had had over 600 calories! (The dressing itself had 300+) If your unsure what to go for, choose something NOT battered or breaded, go for grilled fish or chicken breast, and instead of fries or wedges ask for a side of steamed veggies. For salads ask for vinegaretts instead of dressing, or just eat them plain. Also do your best to have half of your plate filled with veggies.
Also make sure you're drinking plenty of water. Try to aim for between 1-2 liters a day, whatever feels natural. And keep in mind that liquids like milk and juice, while good for you, need to be in moderation. Both can have over 100 calories in just 1 cup. So drink up, but remember everything in moderation.
As far as exercise goes, try joining a school sport. There's lot of activities out there and they can be really fun even if the team stinks. If you can't join a sport try walking from a bus stop further than where you normally get off or walking home from school if you don't life too far. If you have a dog this is a good opportunity to get both of you fitter. Just remember, be safe- wear easy to see clothes if you're near a road and avoid sketchy parts of town or walk with a friend. Another thing you can do is pretend jump rope. This can be done easily in your room, just act like you have a rope and skip. The goal for starting out is really just to get your heart rate going. Try to get about 30 minutes for 5-6 days a week.
also keep your tummy filled snack in between , getting hungry makes one run for a candy bar or a bag of chips .instead as ducky says , apples , carrots even a handful of nuts, other fruits ..