But do they work at all? Yes, a lot of them do work, but only for the short term. Once you quit, the weight will eventually come back, a couple of weeks or a month later (been there, done that).
Why? Because you don't maintain the "diet". Once you go back to your normal eating patterns and former lifestyle, you gain back the weight you lost. You have to make a lifelong change to maintain it.
Fad diets don't teach you how to do that. Fad diets or lose weight fast diets aren't designed to lose a "lot" of weight, are not designed to use over long periods of time, and are not designed with your body's overall health in mind. Many of them are not safe and can cause health issues that you may find much worse than being overweight.
What happens when you go on a quick diet (fad diet) is you lose some weight really fast. It feels motivating, right? But it won't take long before you get sick of it, or cheat on it, and once you start cheating it won't be long before the "diet" is a thing of the past. Before you know it, your weight lost is found again.
So you find yourself looking for another fast diet to try - the magic ONE that's going to work all the time. And you lose some weight. A few months or a year later, you're back up in weight and looking for the next diet.
There's a name for that. It's called "yo-yoing". Up, down, up down. Do you really want to live that way? Depriving yourself of the foods you love for period of time to lose weight? Well, you can lose weight just fine eating every type of food you love. You just have to learn how. That takes time and effort.
I seriously urge you to find a healthy plan, and start making a long-term lifestyle change instead. See your doctor. Change your eating habits and your exercise habits. It's better for your body, for your emotional health, and ... for your pocketbook.
Fad diets will not make you "lose that belly fat" no matter what they say.
What makes it really hard for those who aren't experts (that includes me by the way - I'm not an expert, just a little more aware than some due to long-term experience) is the proliferation of really bad advice on the web. If you do a web search for weight loss plans, you'll find some really good ones, and some really bad ones and some really popular ones.
Just because a plan or program is popular doesn't make it "good", or "good for you". Overall, it might be a healthy plan, but to make it work for YOU, it has to be something you can do, personally.
I'm not going to tell you what plan to choose. I don't know which one will suit your lifestyle. Only you know that.
But (hahaha, there's always a "but") you should choose one that includes all food groups*, choose one that teaches you what portion sizes you should use, and how to count calories (sorry, but yes, that's important), one that provides some sample menus you can cook yourself, one that lets you eat out, and one that teaches you how the food nutrients work with your body (calories, carbs, fats, proteins, etc.)
*Obviously for some, "all" food groups may not fit their health requirements (those with food allergies, vegan, vegetarian, celiac sufferers, those with gluten intolerance, etc.) and in those cases, start with your family doctor. Ask them to point you in the right direction, or refer you to someone who specializes in the area you need help with.Choose one that fits your budget (some might even be free).
Then once you get started on your new food plan, look for a fitness plan. You don't have to look like a bodybuilder, or run 5 miles a day to start being more fit and active.
If you want to start out slow, try something like Zumba classes once or twice a week. Or water exercise classes, or ride a bike. Whatever you think you'll do and stick to.
Later, you might want to look for a gym. If you want to start a weight training program, you should start with a certified trainer, for at least a few sessions (6-8 weeks would be a good start, and it isn't every day usually - work it out with your trainer), otherwise, you can do yourself more harm than good.
The bottom line is, if you want off the diet merry-go-round, you have to change your lifestyle. There really aren't any other good options, and there isn't anyone in the world who can make you do it.
Whether you take that step is only something you can decide.