Before you can begin writing your essay, you need to determine a couple of key pieces of information. First, you need to decide on what type of essay are you are going to write. Based on your topic, I anticipate you need to write an informative essay or a persuasive essay. The former will be a bunch of facts strung together in sentence form. You, the writer will need to stay as neutral as possible while providing information on fast food nutrition. If you are writing a persuasive essay, then you need to do decide what side of the argument you favor more. Is fast food okay to eat or not? Support either side with facts.
The other thing that you have to decide about your essay is who your audience is. You will write very differently, if you are writing to an elementary student vs. a peer of yours. Your audience's prior knowledge on your subject will determine a lot of what you have to write about.
I recommend going with a persuasive essay. Start your first paragraph with an "attention getting" statement. Make it a bold statement of opinion. Ask your reader a question. Give a definition. The point of that first sentence is to hook your reader on your topic. Hooked readers keep reading. That's the primary goal of your first sentence.
The last sentence of your first paragraph needs to steer your reader toward the point that you are trying to make. This is called the thesis statement. You could write something like "Although fast food generally has a bad name in the world of nutrition, it's actually healthier than most people realize." I have no idea what your research says, but that thesis gives you the opportunity to discuss both sides of the argument. You admit to opponent's arguments, but you then convince them that your opinion is better based on your facts.
The second paragraph will be the first paragraph of support for your thesis statement. Be sure that it is clear from the first sentence of the second paragraph that it relates back to the thesis statement.