Hey there,
The main piece of advice I can give you is to make it clear that this won't go away overnight. You can run 15 miles a day every day for a year, swim your heart out and lift weights till your blue in the face... and whilst you may lose weight, it actually won't be as much as if you take the slow and steady approach because if you do overdo it, you will DO YOURSELF DAMAGE.
So, what can I suggest to start? Well Running is a great form of cardio exercise, and cardio is the best way to raise the metabolism (and hence shift more fat off you) but be sure to run at a pace that you can maintain. If you try to keep up with those who are twice as fit as you, it will really knock you for 6, and do so much damage to your muscles that you can't do it another day. Remember: especially as your growing up, not doing permanent damage is more important than how long this takes. I know you'd like to have a girlfriend and now, but seriously, they'll be plenty around a few months later on, and when you achieve your goal, they'll think twice before calling you ugly
So, when running, look to be able to run or jog whilst still being able to talk to your partner, even if panting slightly. I don't know if you've run before, but the first few weeks it SERIOUSLY KILLS. But the reward is so worth it; so get your friends to egg you on and keep at it, it will hurt but it will be worth it.
Other tips... don't work out with your weights every single day. Do it like, 4 times a week, twice to start with. When you work out properly, your effectively damaging your muscles so they can be repaired again and made stronger. This is fine as long as you don't do too much damage and it takes too long.
Another danger of working out when you have slightly too much fat is you get the development of 'fatty muscle', where the muscle tissue expands and includes fat as well as lean mass. This is usually not desirable; therefore I suggest instead of working out doing say, 10 reps of the highest weight you can handle, do 2 sets of 30 reps of the next two down... reps increase the toning of your muscles instead of damaging them beyond repair. Either way working out raises the metabolism and burns energy at the same time; its great.
Also, whether you change your diet drastically to begin with or not, DRINK WATER. And plenty. Some say up to 8 litres a day, I say switch slowly and drink as much as you can, avoiding unhealthy stuff like coke and alcohol (hey, your parents could let you drink it! )
I'd also suggest swimming as a great way to start to lose weight... because of the minimised resistance in the water, I find that you get to use your muscles therefore burning calories for the least effort - something which can be difficult to find in the early stages. We all begin with good intentions but sticking to them is what this site is all about, so of course, tell us how you get on. The people here are helpful and nice to be around... I'm sure they'll help
Best wishes with your goal,
Arron