Sweety, you CAN do this!!! I know it's hard to get started and stick with it, but it's definately worth it.
I lost 40-50lbs. before going into high school, but I gained it all back, plus another 70 lbs. I really looked at myself one day, looked at pictures of myself, and I was totally disgusted with what I saw. And then it just hit me, I HAD to do this.
I had no clue what to do when I first started. The first thing I did was buy some "beginner" exercise dvd's. Extreme Makeover Fitness is EXCELLENT. The cardio routines are only 10 minutes each, and they have different routines for each day of the week, Monday through Sunday. There's strength training too. They also show you low-impact moves. All you need is a chair. The second thing I did was join fitday.com (it's free). It lets you enter ALL the foods that you ate in one day, plus all the activities and exercises that you did. Then, depending on your body weight which you enter when you join, it calculates the calories that you took in and the calories that you burned off. It was a great way to start a food journal. I also started a regular journal, writing down my motivations, why I want to lose weight, etc. I looked at this often. I started measuring out serving sizes until I could "eye-ball" it. I eat a big salad before dinner so I don't eat as much. It really becomes habit after a while and now I feel sick and get the "runs" if I eat anything greasy or whatnot.
My first week, I lost 8 pounds and in less than 3 months I've lost 61 pounds. I haven't starved myself, I never did any of the "fad" diets, never joined a gym, etc.
As for exercising, you don't need to go to the gym, you don't need to buy a treadmill or any other exercise equipment. The outdoors is your gym! Start off by walking. Get a pedometer so you can see how far you've walked. Read books on dieting and nutrition...I did a lot of that in the beginning.
Most of all, you HAVE to think of this as a life-style change...NOT a diet. A diet is something that is only temporary, and I hate to think of myself as being on a diet for life. Remember, it is OK if you overeat one day, or don't exercise one day. You just jump right back on the next day. I used to freak out if I ate anything that was even REMOTELY bad for me (fatty or high in calories)...thought that I would automatically gain like 5 pounds back. But, obviously that never happened.
Have faith in yourself. It is attainable!!