teen starting fitness regimen?

I'm around 5"2, 15 years old and 120 pounds. I definitely don't have an ED or anything but I have a very high fat percentage (almost borderline in the unhealthy/overweight zone) 

Basically I eat a WHOLE lot of junk and binge quite a bit whenever I feel like it (think an entire meal, tons of ice cream, some potato chips and other snacks) LATE at night heh. Which I'm trying to change!

The irony is that I'm CONSIDERED quite fit and used to be in a variety of sports like in sprints for the track team and judo as well, gaining quite a bit of muscle in both, but they're all turning into fat right now because I don't do sports regularly anymore.

I would like to get rid of my huge prominent tummy bulge, get rid of my fat thunder thighs (genetic!!!!!) and tone up my arms - i know i have quite a bit of muscle on them, but they're turning into fat more and more.

I have a knee problem (which I suspect is an old injury from track, because my coach told me a couple of times that I used my knee too much) and I haven't officially went to a doctor to get my knee diagnosed (because it hurts only when I do things like run a couple of rounds during physical education or exert it somehow) and its fine normally but I think there's definitely a problem with it. Could it be that my weight is too much for my knees to support?

I've worked out on treadmills for around 30mins each time and to be frank I really DIDN'T ENJOY IT AT ALL as they made my knees hurt sometimes and found I prefer the elliptical machine. And to be safe I'd rather not risk my knees in case.

I'm pretty clueless about fitness actually and how to lose fat. My ideal body shape is LEAN with muscle but not overly muscular but I'm wondering should I do cardio then work on the individual parts - like crunches for my tummy, the leg machines and weights for arms? And HOW many minutes exactly do I need to do cardio for to burn fat effectively - I'm not that concerned about weight right now, just the borderline overweight fat percentage in my body.

My mom is also planning to get an exercise machine and I told her elliptical machine - should this be the one for me?

Thank you so much for your help in advance to a clueless person like me!

There's no way you are going to get any noticeable results without cleaning up your diet. Stop eating garbage and cut the binging before you worry about your training program.

and your muscles probraly arent turning to fat its just covering up your muscles clean up your diet start weight lifting and cardio

Muscle CANNOT turn to fat, they are two seperate tissues, it isn't possible for one to turn into the other.

Weight lifting is the best form of exercise to burn fat, followed by high intensity interval training (which can be done on an eliptical).

And to reiterate, diet is the most important factor in gaining/losing fat.

oh yup so sorry just to clarify, i AM clearing up my diet already :) eating lots of boiled foods now heh heh. just wondering what is the next step? and do i have to take more protein to build muscle cause i tend to prefer eating vegetables and fruits as opposed to meat?

We don't have enough information to recommend eating more/less of anything.  What does a normal day's diet for you look like? How many calories are you consuming? how many are you burning? what is your macronutrient breakdown (% protein, % carbs, % fats)?

With the body composition analyzer in school I've been told my BMR is around 1300 i think (not exactly sure)

I'm not too sure how many calories the activities I take part in after school burn, but I have fairly intensive dance/cheerleading on one or two days a week and a hiphop class every week. 

After counting calories, I realised that I might have been eating too little for the past few days (around 600-700) cause I've been eating mostly fruits but the analysis for those days are:

Fat - 19.4% (61 grams)
Protein - 21.0% (149 grams)
Carbohydrates - 59.5% (422 grams)
Alcohol - 0.0%
Other - 0.0%

You definatly need to eat a LOT more, if your BMR is 1300 (which sounds on the low side to me) you're probably burning over 1500 a day without exercising. 

your percentages are good. "ideally" the percentages are 60% for carbs, 30% for fat, and 10% for protein. you dont necessarily have to eat more protein to build muscle, just eat it at the right times, like snacking on something 'proteiny' during your workout or/and eating something proteiny after your workout. and 600 cals will not, uhm, do anything for you if u want to build muscle or be healthy. you gotta be eating healthily to get healthy, that should go without saying. you cant starve yourself to a healthier you.

strength training / weight lifting is the best way to build muscle and burn fat.

Shes right, if your trying to gain weight and muscle, carbs are mostly what your looking for.