I can give you the base of your essay:
1) fast food companies target low income people. And because the price is so cheap and convenient, people tend to buy more.
2) It’s affordable, and you can be in and out in 5 minutes with enough food to fill you up for a meal.
3) fast food is often made with cheaper ingredients such as high fat meat, refined grains, and added sugar and fats, instead of nutritious ingredients such as lean meats, whole grains, fresh fruits, and vegetables
hope you do good in your essay :)