You sound just like me when I started. I had about 120 pounds to lose when I started, and it seemed overwhelming.
I think that the key to sticking with something more than just a couple of weeks is to not make any radical changes in your life, at least not to start. I know, it sounds counter-intuitive. I mean, you need to lose 100 pounds, surely you need to turn your life upside down, right? No. I started by making small changes, things that I could see my self doing for a long time (as long as I don't want to regain the weight, which is forever). You can turn your life inside out for a little while, but soon your old habits will re-assert themselves. Gradually develop new habits.
Losing weight is simply a matter of eating few calories than you burn off. That's it. You can count carbs, protien, or fat grams all you want, but it does ultimately boil down to the calories. You simply have to eat less, there are no magic formulas or diets or whatever.
I think it is important to know how much you really are eating now. One of the first things I did was buy a digital scale, and then use to learn what I thought a serving of something was, versus what a serving really was. If you are accustomed to eating a big bowl of cereal every morning, you might be surprised to to find you are eating 3 or 4 servings of the stuff. Many people find that using an online calorie counting program is very helpful.
www.fitday.com has one that is free, and you can add you own foods to it so that the more you use it, the easier it is.
Don't ban any food, especially stuff that you really love. If I told myself that I couldn't ever have barbequed ribs, I wouldn't have gotten very far. Just understand that you can't have whatever it is you want as often as you might want it. I want some ribs now, but instead I'll have a small, sensible dinner, and perhaps on the weekend I'll have the ribs.
Learn to make healthful substitutions for what you normally eat. Do you love french fries? Try oven roasted fries instead, or even sweet potato oven fries. Love fast food hamburgers? Give home made veggies burgers a go. Gotta have dessert? Try sugarfree angel food cake, or fat free pudding a try.
Read the blogs and websites of other people who have had some success. See what they say they are doing, and then try some of the same things yourself.
Try exercise. When I first started, I commited to just ten minutes a day, 5 times a week. Ten minutes? I could do that! That was just a walk around the block a couple of times. Heck, I could almost do that during a commercial break. Even this little bit helped, and soon I was doing more just because I wanted to.
Understand that it will be a slow process, and you might not see the changes in yourself for a while. I lost 30 pounds before other noticed, and close to 50 pounds before I could see a change.
Understand that you might be hungry from time to time. It's okay, hunger isn't the end of the world. Don't let yourself get starved to the point where you are ravenous and don't care about anything other than eating, though.
I hope this is enough to give you some ideas on starting. I'm sure you'll get lots of great advice here, I know I sure did/do!