Hi there,
There is a lot of good advice in this thread, I think. Let me relate what worked for me.
I've lost 133 lbs. over the course of two years. I started at 300 lbs. on the dot and am currently 167 lbs. Lots of people ask me how I did it; what diet was I on? Did I have surgery? etc.
No surgery. No strict diet per se; just my own "diet" based on dietary theories I have read, from low-fat to low-carb to low-calorie. In the end, my principles lead to a reasonably a low-carb, low-calorie, and low-fat diet.
My principles:
1. No refined sugars, or at least very little. Natural sugars in fresh fruits and veggies are fine.
2. No bread, no rice, no pasta, no potatoes. That really limits complex carbs. By "no" I mean six days a week none at all, and when i do have them, it's a small serving.
3. Eat more fish and chicken. Lean proteins are good. Red meat occasionally, but more on the fish/chicken side.
4. Eat more veggies, espcially fresh veggies! And fresh fruits, too.
5. Moderation! Don't go nuts on anything (like eat two steaks because you're on Atkins or half a jar of peanuts or whatever).
6. Exercise! I exercise quite a bit naturally because I like sports (golf, tennis, basketball, and biking). Beyond those fun activites, I have a regimented routine that I follow at least three times a week (treadmill, cross-trainer, and light weights at the local YMCA). Do serious aerobic exercise (that is, work and sweat) for at least 30 solid minutes. You may have to work up to that; when I started, I could barely stay on the treadmill for 15 minutes at a low speed and no incline. Now, I am walk as fast I can without kicking into a jog for 40 minutes at the highest incline level on the treadmill. As you work more, you'll be able to go longer and harder, and you just keep pushing yourself each day.
Note that I follow these rules when I am what I call "weight loss mode." That is, if I'm trying to lose weight. If I want to maintain weight, I eat more a lot more like a typical person; poatatoes, pasta, bread, rice are all back on the menu, I just watch the portions. I try to follow the rule of one: one serving of anything is OK; two is not. I can maintain my weight and eat what I want, then, in moderation, as long as I keep exercising.
For the last couple of years, I've been in "weight loss mode" on and off; I let myself go into maintenance mode from November through January (birthday, Thanksgiving, Christmas, New Year's, Super Bowl) and July-August (4th of July, Origins, GEN CON, summer fun.)
I have almost reached my final goal, which is 150 lbs; half the weight I was. Mind you, it has taken years, not weeks or months. Losing more than 2 lbs. a week is not good over long periods.
Part of my success may be that I haven't always been heavy; in high school, I was 135 lbs. (I am not tall-5'5"). So I knew I could do it; I just let myself go weight-wise after high school.
BTW, I do drink diet soda in moderation. And it is usually caffeine-free. Even when I'm not in "weight-loss" mode, I now prefer diet soda to regular, which now tastes way too sweet for me.
Good luck!