first of all, good on you for realizing you need to do something about your weight & health. When i was your age (im now 22) i was obese, but i saw myself as normal. i wish i had someone to tell me 'hey, you need to do something here'. so good on you for getting started while your still very young.
at your age, im guessing your either still in high school, or maybe have a part time job or something, what are you up to at the moment? how much money can you afford to spend on your health? do you have enough to get a gym membership, or enough just for some new walking shoes? work out a budget so you can work out how you will start exercising, at a cost(gym) or for free( walking around the neighbourhood).
hows your diet? do you still live at home, or living on your own or with friends? how much money can you afford to spend on quality healthy foods? if your strapped for cash let me know and ill give you some tips of budget healthfood shopping.
come up with some goals. how much do you currently weigh? what is your height? what is your goal weight? break down your total weightloss goal into smaller mini goals and work at each goal one at a time. include non-food rewards when you reach these goals, either a new outfit or haircut etc.
work out some motivation. is there anyone you aspire to? me personally i aspire to rihanna & jessica alba, because they are both very fit & work very hard at achieving their looks. this forum is great for inspiration- read some of the true stories and before/after pics.
a good way to get started and keep true to your goals is to keep a journal. i keep a blog- http://jessica-mole.livejournal.com/
its a great way to put your goals, thoughts & feelings along with a diary of your diet/exercise habits so you can reflect and learn from your weight loss journey.
why not start a challenge? there are sooooooooooooo many challenges on this forum, or you could make up your own.
hope this helps, and look forward to hearing how you are going.