I don't think that you necessarily have to go visit two doctors to get this all figured out. I know it might be hard, but do try to talk to one of your parents about it. It would be much better to have someone on your side (especially if they do the cooking around home) and to have support as you go through this.
If you have a family doctor, I would recommend going to them. I know that you want to try to do things on your own, but they can help you determine some healthy things for you to be doing (everyone is different), and cocogirl is right, it could be hormonal levels. Something like hypo/hyperthyroidism or anything like that can really shift around your metabolism and affect your weight and energy. Things like that aren't a quick fix and may need to be medicated - I know, b/c my boyfriend has hypothyroidism... If you can't do that, maybe just try the school nurse. They have tons of information and i'm sure would be glad to help you get started.
As far as some advice, it all depends on what you want to do. I would suggest eating healthy - lots of veggies, whole grains, good amounts of protein. Variety is good! I typically try to eat between 1200-1400 calories per day, and also eating small meals throughout the day has helped. Small breakfast, a piece of fruit or granola bar around 10, lunch, snack around 3, and dinner. I don't recommend Atkins, because you aren't getting a good balance of all the food groups and while it seems to be a quick fix, the weight comes back and you don't stay healthy.
Exercise will totally help anything you do. there are lots of suggestions posted in the forums, find something you like and will get excited about doing, and do it =) Remember, this isn't going to be an overnight process. This is a journey, and we are here to help you!
Welcome to this forum, and I hope that you can find the support to keep positive about who you are and where you are headed =)