Wow! I definitely like how you've thought everything through and are trying to do something about your weight at such a young age. I mean I'm only 19 and I am also trying to lose weight. It will not only help us feel better about our image and make our bodies feel better, but also be more confident and to stay focused to try to keep that goal weight throughout the years.
I think your goal so far is good. Running is one of the best steps you can take if you can do it, it helps with your legs, like inner and outer thighs, even your stomach and overall body. Sit ups are good as well...
As for me, just to let you know of what I do, I bought a 20-minute workout video I try to do at least 4 times a week. it's like a strengthening video too which helps with balance, and overall structure and posture. I also have one of those workout balls, those big ones. I usually sit and bounce on it a while even while watching TV. Also I take some 8-10 pound weights and lift them for my arms. It's great to do sit ups and push ups on the ball as well. They are not even that expensive either! Under $10, so finding those things that help you for an inexpensive price is always wonderful!
As Spring and Summer arrive, I will walk/run more often to get used to that. As far as my diet goes, I've learned to absolutely love salad. Which is good, maybe put some vegetables in there as well with little dressing.. perhaps some fat free dressing. I work at Olive Garden and their italian dressing tastes so good and isn't many calories, so definitely find something that you like along those lines. For dinner, some lean meats even like grilled chicken is really good for you. Add some vegetables that you like, and that will make it easy if you go out to eat. Also, eating less for your meals will make your stomach shrink a little so it will be easier to get fuller faster without eating a lot! Depends on what you're used to and how much you would like to change. For breakfast, I eat the Special K cereal, and usually I eat 1-2 snacks a day, that way when I eat my main meals such as lunch and supper, I won't eat as much because I will not be as hungry from my snack. Snacks such as: an organic granola bar.. my favorite is from Cascadian Farms, you can get that at any grocery store basically. Try the chocolate chip ones. Theres not too many calories in that either! Otherwise fruits are good, I love berries. Even eating a banana or an orange too, it's just right. Nuts are really good too, they're nutritious and helpful to lose weight. Especially walnuts! I just recently read that they have the most antioxidants in any nut and has fiber so you can have that for snack and eat not as much.
I really hope this helps! Not only just from my experience, but I also think that you are on the right path to success. Like I said, I'm not even out of my teens yet so I can relate to you as far as weight and the goals you have are similar to mine.
One more thing... seriously, if you like dark chocolate, awesome. It's not only good for your heart, but if you eat a little 1-2 pieces of it a day, it'll help make you not eat as much for meals and help you lose weight! Oh and that reminds me.. drink ice cold water and put some lemon in their as well. The citrus flavor of that has anti-oxidants as well and will help you lose weight.
Just make sure to stick to exercise so that you keep the weight off. Don't be afraid to splurge once in a while though, perhaps once a week.. if you crave something, eat it. Otherwise, you will be eating little things all day which will probably add up to more calories that what it is that you're craving for.
Good luck with everything :)