I second josie’s comment. Consider it a lifestyle change, not a “diet” which implies that is is temporary (and which is why most people regain any weight they’ve lost).
Start by drinking a glass of water 10–15 minutes before a meal. Not only will this help fill your stomach a bit, but it will help with digestion and overall hydration, which many of us are lacking.
Next, you must consciously reduce your portion sizes. From Wikipedia: “In adult humans, the stomach has a relaxed, near empty volume of about 45 to 75 ml. Because it is a distensible organ, it normally expands to hold about one litre of food, but can hold as much as two to three litres.” Think about a 1-liter soda bottle, which is a little bit more than a quart. That is about how much you’re supposed to eat. Try not to exceed that amount in a single meal, and your stomach will gradually shrink so you will feel more full with smaller portions.
Eating more slowly and deliberately helps you truly enjoy the sensory experience of eating, so you fee satisfied with less. Too often, we eat in front of the television or computer, so we aren’t really aware of what and how much we’re consuming and we eat mindlessly. Turn off the electronics, other than perhaps some background music, and immerse yourself in the sight, smells, tastes, textures, and sounds of your meal.
Control the proportions of your food, so that fresh vegetables are the bulk of the meal, lean protein is a smaller amount, and starches are a small side if you can’t leave them out completely.
Use progressively less added fats, sugars, and salts, and if you use dairy products, wean yourself down to lower fat products. Be careful, though, because some products add sweeteners to compensate for the lower fat, such as coffee creamers.
The same goes for increasing your activity. Start with little things, like parking in the back of the lot, taking stairs instead of elevators and escalators, going for a walk around the block after dinner, etc. As you start to feel a bit better, you may want to add new activities, such as swimming or biking, or maybe go on some nature hikes. The key is to find activities that you actually enjoy. See if there are groups you can join in your area, through your local community organizations or maybe on Meetup.com.
I’ve always said that losing weight is the hardest addiction, because while one can give up drugs, alcohol and smoking cold-turkey, one must continue to eat. You have to really want it in order to make it happen. I hope you find something that works for you!