1. Keep a food journal - someone has already recommended My Fitness Pal - use it!! Great tool.
2. Weigh ALL of your food, don't trust that your 1/4 cup measuring cup is the same as the food manufacturers 1/4 cup, go by grams or ounces instead
3. Make your changes in small steps and slowly - this is a lifetime journey as someone else has mentioned.
4. Realize that food does not equal comfort. When you find yourself reaching for a package of cookies because you had a rough day, sit down with a pen and notebook and journal about why you feel like gorging on comfort food.
5. Be consistent with exercise and realize that this is something you also have to make a lifetime journey - both for physical and mental health. But play around - swim, bike, run, hike - just change your lifestyle from a sedentary one to an active one. Talk your friends into being active with you or seek out new, active friends.
6. Eat "close to the earth" - I've heard this phrased so many different ways but it is so true. As little processed food as possible. An occasional treat is absolutely alright, but plan for those treats. My partner and I have a rule - we can have a donut if we run 20 miles. I've had to change that a bit lately as I've been biking instead of running, so my personal rule is now a soft pretzel if I bike 30. The funny thing is though, when I hit that mileage I often don't want the treat because I feel so good about what I've accomplished!
See if you can re-frame your goal from an arbitrary number to a mindset. The number you have in mind may or may not be the right number for your body (I learned this the hard way!) Make your goal to be healthy and fit instead of a number. As someone else said, you have youth on your side so you are SO lucky to be starting this journey now and you are so smart to be seeking out advice to do it safely. I commend you on every one of those - you are way ahead of the 8-ball!
Congratulations on taking your life into your hands!