What is important is that when you eat, you should actually like the food. If your eating something because its low-cal, but you dont like it, sooner or later you will go off your diet. Experiment with different foods. If your eating with the family, and they dont make the healthiest meals, learn to cook and get involved so you can have more of a say in whats being eaten.
A very important note is this, try to consume zero calories through general beverages. If you have been drinking soda and iced tea, and find it hard to go straight to water only, buy some lemons, squeeze them, add to water with some splenda for some lemonade. I did that for several months before I finally transitioned over to water completely. I have skim milk now and again with a peanut butter sandwhich, but not often.
Also, if you have a job around town, house or wherever, save up a little money and buy yourself a heart rate monitor. Once I got one of those, it really took my exercise to the next level. Once I knew where I should be, I realized that I was often falling short of how intense a workout I could reach. Dont have to have one, but if you get the extra cash, it could come in handy. You could save up for a better bike as well. At 15, you dont always have a ton of options in the cash department.
you may want to find a friend to do this with, if you have any in your situation. having a buddy to get into eating right and exercising helps a lot of people stay on course.