Since this sounds like a short, open-ended topic from what you've described, you may want to consider starting with what you know about this topic personally as a first step.
For instance, consider what you already know or believe about fast food. Do you eat it sometimes? Do you know others who do?
Can you think of any impact it has had on your family or can you picture how and when families turn to fast food? Be sure to consider both positive and negative sides of this.
For example, what can you imagine is a benefit of having available fast food in close proximity to a person's home? What are the downsides?
Do any of those benefits or downsides relate to the way a family functions, such as the closeness of their relationships and the health and well-being of family members?
Once you've considered what you already know through your own experience, it's time to put your beliefs and ideas to the test. Other responders to your question have mentioned good resources for this.
Of course, you'll want to consider more than your own ingrained thoughts on the topic and discover what others think to make your essay complete. Yet, starting with your own experiences and perspective can help you to get rolling and make this assignment more interesting.
The topic of "how fast food affects families" is an interesting one, but you need to decide what you want to say about those effects on families. Other things you must address include for whom you are writing (audience) and the purpose of the essay. Are you writing to persuade your readers or simply trying to compare/contrast the negative and positive affects of eating fast food? Knowing these things will make writing your essay much easier, so let's get started.
Since you already have your topic, you need to decide what you want to say about it. To help you determine that, try generating some ideas through a prewriting exercise. Without letting your pen or pencil stop, write down everything that pops into your mind about the topic. For five minutes, simply jot down words and phrases, and don't worry about proper grammar or mechanics at this stage. If you find you become stuck, rewrite the last idea over again and again until something new comes to light. Keep writing throughout the entire five minutes. Once you're through, organize similar ideas together. You should begin to see things that stand out more frequently or forcefully than others. From here you should form a question or two to help you create a thesis statement. This thesis will give you the direction you want your writing to go--your purpose.
The next step is to determine your audience--to whom you are writing. Are you writing to your teacher just to complete this assignment? Are you writing to parents to convince them that eating fast food does have its merits? Are you writing to your classmates to educate them on how eating too much fast food can inhibit their athletic performance or possibly lead to obesity? Are you writing to a general audience to educate them on which fast foods are healthier options? Of course, many of these decisions are influenced by the assignment specifics--particularly the genre of essay.
Your instructor may want you to use a specific rhetorical strategy. Are you working on comparing/contrasting information? Is the essay supposed to persuade your reader into making a change? Are you describing the harmful results associated with consuming fast food? Be certain to clarify the format you are to use before you begin your research for support material.
Now that you know to whom you are writing, what you want to say about the topic, and the genre in which you will be writing, you need to look for credible support material. You can find nutritional information about the food served at many of the large chains simply by Googling the restaurant names. Google your thesis statement to see what information appears. Try looking at sites such as the American Heart Association or the American Cancer Society to see if they provide a correlation between fast food and major diseases. Remember that Web sites ending in org, gov, or edu tend to be much more credible than someone's blog or Wiki post; so be very selective.
Always remember that writing is a process that involves prewriting, writing, and revising. A well-written essay takes times to produce. Therefore, be very conscientious about completing each phase of the process. By doing so, you will have more success in relaying your message to your audience as you intended.
I'm not sure from the question whether this is a research-based essay or not, and what sort of sources you need to use. Fast Food Nation: The Dark Side of the All-American Meal by Eric Schlosser, summarized on eNotes, would be a good starting point for a research-based essay.
The first issue you might want to address is the effect of fast food on families' health. You could analyze the nutritional quality of a fast food meal by going to the websites of popular fast food companies such as McDonald's or KFC to discover the nutritional content of their meals, including calories, fat, and sodium. Then you could vcompare them with the healthy eating recommendations of the Centers for Disease Control and Prevention.
The next part of your essay might look at the sociological issues of fast food. A traditional family dinner, in which parents and children sit together over a meal and talk, increases family cohesion and actually helps socialize children into adult life, something that does not happen when family members eat fast food while watching television or concentrating on their own electronic devices.