I don't know much about fitness and the like, but my parents are crazy on it so I know some stuff..
Consider signing up to something like 'Dietchef' or 'WeightWatchers'. They can help you with what food to eat, how many calories you should/shouldn't intake in a day etc etc.
For exercise, just join your local Gym or any good Gym for that matter that you can get your hands on. From there you will have to make a membership which is normally paid monthly. For doing this, you SHOULD get access to a swimming pool and a variety of exercise equipment such as weights, treadmills, exercise balls, and those weird bike machines (sorry, I dunno what they're called, but they're basically a bike version of treadmills ;P).
In regards to what size you wanna be, I can't really advise you on that. However, if you do decide to join a Gym (which you SHOULD imo, as it can help immensely), there will most likely be some professionals/trainers that can help you with that. For example, in my local Boots there is a machine which measures your BMI (Body Mass Index), your height, and your weight and then informs you what you should be at. It's a very useful piece of equipment.
However, I could be completely wrong, but that's just my view on things since my nan (my NAN, who is 57) followed a plan like that and lost 12 stone. I've seen some pictures of her before, and trust me, compared to a 57 year old losing 12 stone, a 17 year old losing 9Kg should be an easy feat.
But anyway, good luck with everything and I hope it all works out :)