Hey congrats! It's a harder battle than most people think.
When I was in elementary school I was a little pudgy. I swam competitively and I was the chubbiest girl on the team.
Strolling down memory lane is depressing me... Oh man, where's the ice cream?!?!?!
But seriously, my parents took me to a nutritionist and helped me live a healthier lifestyle. I slipped up a bit my first year of university... meh, the freshman fifteen, but then I started training for triathalons and that balanced out the beer and pizza diet.
I find moderation is key. You can't deprive yourself of the things you love and over-indulging obviously is not healthy either. It's both excercise and diet together and focusing just on one or the other will make it more difficult. I used to think that if I just don't eat anything, it'll be okay, but I'ld rather run an extra mile than not eat. I also found that while routine plays a huge roll in maintaining a healthy lifestyle, it's also important to try different things so you don't get bored with your routine. Sometimes I take dance classes, boxing, touch football, baseball, yoga, whatever. Plus, if you have friends/family that are willing to do these activities with you, it totally helps. While in university my buddy and I would go to the gym together and that motivated me to go more often, because she would more or less make me when I wasn't in the mood, and vice versa.
When it comes down to it, you yourself need to have the want to change, or it will not work. Nobody can force you to do it, you have to make the commitment to yourself. For me the catalyst was getting back my swim team pictures in grade 6, and while looking back now, I was kind of a cute chubby kid, but I don't even want to think about the path I could've taken.