Hi Erika and Welcome !!!
Selecting a diet is a personal thing, you need to do what works best for you, even if that means experimenting. The first thing I would suggest b/4 starting any plan is talk to your doctor because he/she know's you best (besides you).
There is one thing I can NOT stress enuf, and that is, bottom line to any diet is calories! You don't necessarily have to "follow" any particular plan, you can make your own up depending on your budget. I follow atkins and I have to say, I don't spend any more $$ now than before I started. It kinda even's itself out because I'm not buying junk food anymore and instead I'm putting my money towards healthier foods.
My sister was close to 400 lbs. By her cutting down on her intake of food "calories" and she cut out most if not all of carbs such as, pasta, breads, rices, stuff like that, she eats lots of salads and veggies, fruits, etc and so far she has managed to lose almost 100 lbs in 16 months. And she is still going strong. My point is, you, like her can customize a plan that works for you.
Weight Watchers also has a great Low Carb plan. South Beach, Sugar Busters, Atkins.... they are all wonderful. Don't just jump on the band wagon of the most popular diet, choose what you think would work for you. What have you done in the past that has worked ? Sit and think about it. Write it down.
Good for you for getting back to exercise, its one of the most important things you can do for your body, weather or not you lose weight, its a good healthy habbit to have (its good for your bones too

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Leenie