Fitness is a lifestyle. That is something that I truly believe in the most literal way possible. A person should not go on a diet, they should eat healthy all the time. A person should not start exercising in order to lose weight, they should exercise everyday in order to be healthy. If you look at the concept of fitness as temporary state.
As in "I am getting fit in order to lose weight" or "I am getting fit in order to impress that girl or guy" then you are missing the point of fitness entirely. So let's put aside any notions of quick and easy dieting, or fad exercise programs, or any other silliness. Instead let's focus on what works and make a lifestyle change that will empower us for the rest of our lives.
It all starts with your mental outlook. When you look at fast food what do you see? Do you know what I see? Calories, fat and a future heart attack. Yes that is being a bit extreme, but what we are trying to do here is rewire the way your brain thinks about different aspects of your life. If you are serious about getting fit and staying that way then the first thing that you have to change is your outlook on fast food.
Fast food it not nutrious in anyway. In fact it is just the opposite. A typical fast food meal is loaded with enough calories to feed you for the entire day. Normally if a person were to eat three fast food meals a day (breakfast, lunch, and dinner) they would consume between 5,000 - 9,000 calories depending on their choices. Keep in mind that the average male needs about 2,200 calories throughout the day (assuming that they are somewhat active) and the average female needs around 1,800.
I hope you can see the extreme problem that you would face if you made eating this type of food a normal part of your life. Losing weight and keeping it off is virtually impossible if you are always eating this junk food. Besides it is just too easy to prepare really healthy foods that taste pretty good to waste time with this junk.
For example you could replace that cheeseburger with a nice fish filet. A full tilapia filet is only about 100 calories. A full salmon filet is about 390. A salmon filet has more fat in it, but it is good fat. The fat you get from a cheeseburger and fries is all bad. The trick to making this work is that you really have to internalize the idea that some food is killing you, while other types of food are making you stronger.
Other good sources of healthy protein are eggs, chicken, turkey, and tofu (if you are looking for a vegetarian option).
The point I really want you to embrace here is that the goal of fitness is to enable you to live a healthy life till the day you die, not just to lose a bit of weight.