Shannon, you are here, and that is a good step! ;)
Seriously, just the fact you are here asking for assistance is a good thing. We all need motivation, and I think you are learning that the most effective motivation comes from within, YOU have to want it.
I think, for someone like yourself, balance is the key. You don't need to cut out soda, sweets or that burger, you just need to employ moderation. You also don't need to run 30 miles that first week or climb your local mountain with only 1 liter of water.
Train smarter, not harder. Too often I see people, with the best of intentions, train too hard at the beginning. And what happens? They quit. This is obvious because who wants to suffer? Maybe masochists, but most of us don't fall into that category.
This is where training smarter comes in. Have an OCCASIONAL soda, or treat, but not as a regular part of your diet, more as a reward. Start making healthy treats and food a regular part of your diet. With regards to exercise, start of slowly. You don't want to fall into the old trap of making yourself miserable from over-exercising.
Also, have a realistic view of what a healthy body and lifestyle is. I don't know much about you, but I do know that at 17, many of us are filled with doubt and unrealistic views of what a beautiful and healthy body is. By this I mean, read less Glamour and Seventeen, and more Women's Health and Shape.
And most of all, remember this, consistent actions, yield consistent results. There is no magic to being in or out of shape. Eat and train healthily and you will get the results you desire.
Good luck and do your best Shannon!