If you do everything right, you can lose about 3 lbs a week and still be healthy. And the weight doesn't have to come back, ever. When I was your age I was about 30-40 lbs overweight, but I lost it all and have generally kept it off ever since, with only minor setbacks. Now people always say I'm skinny lol.
Don't do a tuna and water diet. Tuna tends to have a relatively high amount of mercury in it. Your body gets rid of mercury slowly, so eating tuna too often will cause a mercury build-up which is unhealthy. Besides, just eating fish (or any other meat) would cause you to become malnourished. Humans are omnivores, not carnivores or herbivores. Our bodies need a variety of meat, vegetables, fruits, grains, etc. to be healthy.
The most important thing is to eat REAL food. No processed or junky foods. Eat at least one salad each day. You can even add nuts or pieces of chicken for protein. Don't use conventional salad dressing. Add olive oil, pepper, and cajun seasoning (if you like spicy food), all to your taste. This combination tastes really good, and it's also really healthy. Also, and this is very important: Eat about 6 smalls meals throughout the day instead of 3 big meals. Your body will not try to store as much fat this way.
Drink a lot of water. If you can, drink real green tea that your brew at home. Maybe your parents would be interested in making it. You can brew a whole pitcher of green tea and keep it in the fridge.
As for exercise, the best thing you can do to lose weight is to do a HIIT running session every other day. HIIT means High Intensity Interval Training. The whole routine only lasts about 10-15 minutes, but it's VERY effective. Start out by jogging for about 3 mintues. Then for about 20 seconds, sprint as fast as you possibly can. Then slow down to a jog again for another 3 minutes. If you're really exhausted, walking is fine (but jog if you can). But after those 3 minutes, sprint for another 20 seconds. Then do it all one more time. So basically you jog, sprint, jog, sprint, jog, sprint, then you're done. By then end of this you'll be very exhausted. This might be one of the hardest, most exhausting thing you've ever done, but it only lasts like 12 minutes, and you only have to do it once every other day. And it's worth it! It really works. This routine will actually make you lose more weight than just boring walking or jogging for an hour. There are studies proving it.
This all worked for me. You should really try it. Anyway, good luck!