I'm a 17 year old girl, and I've been worrying about my weight since god knows how long. I'm about 5ft 10, but I weigh 80kg. According to BMI calculator I'm overweight. I'm a really sporty person (honestly) and do at least one bit of exercise a day. I'm not that bad at eating either. I really want to lose weight really quickly. Any tips?
If you're a really sporty person then some of that weight could be attributed to muscle, as such the BMI may not be accurate. You may want to look into getting your body fat measured for a more accurate picture.
If you want to loose weight it is as simple as eat less and move more, your eating habits will make up the vast majority of your ability to lose weight.
You say you're not that bad at eating but chances are you are consuming more calories than you think, I advise for a week you take a note of all the calories you are eating (don't change your eating habits, you want an accurate picture) and find out the average number of calories you consume daily. Pretty much guarantee it will be much higher than you expect.
As for losing weight quickly, what matters it losing weight healthily. I 100% recommend downloading the My Fitness Pal app, or using their website to track calories and to figure out what would be the best calorific intake for you to lose weight.
Edit: for a quick way to get an idea of your body fat percentage you can use this: http://www.bmi-calculator.net/body-fat-calculator/