Hey Emily

If you're young, your body should respond quickly to making positive food choices and watching your portion sizes. Try to prepare most of your food at home and eat out less - restaurant food (both sit down and fast food) is typically a much larger portion than a normal person needs and is higher in sodium, calories and fat.
To be a slender, healthy person for your life, concentrate on eating as many "whole" foods as possible (fruits, vegetables, lean protein, low fat dairy, whole grains, healthy carbohydrates - sweet potatoes, quinoa, beans, and healthy fats - avocado, nuts, healthy oils).
Read a lot. Look at labels. Use the internet. Make decisions that work for you and your life. Make a plan that is for YOU and only you.
Some good books to read:
Mindless Eating
Fast Food Nation
Fodo Politics
What to Eat
Omnivore's Dilemma
As someone that started "dieting" when I was 15 and weighed 140 lbs and ended up fatter and fatter and 200 lbs at 35, this is the advice I wish I had gotten and followed. I could have saved myself so many years of feeling like a failure - I always wanted to diet for a short time and then eat "normal." It took me 20 years to realize that "normal" American eating habits (big muffins, large size frappucinos, fast food, pizza) made me heavy and I had to really eat better to live as a thin person. That doesn't mean I never get to eat "good" or "fun" - I try to be about 90% on plan and 10% treat - which works out okay for me.
For late night snacking - declaring the kitchen "closed" at a certain hour might work. Definitely brushing/flossing is a good idea. Stay busy - find a habit that keeps your hands occupied like knitting (seriously - you could make a cool scarf!). Call your grandma, write a letter, heck, post here! Drink herbal teas.