How can I start a five-paragraph cause and effect essay on the topic of obesity?

To start any kind of essay like this you need to figure out what point you want to make to your readers.  You want to be able to summarize your point in a single sentence.  Teachers often refer to this sentence as the thesis statement.  It should be placed at the end of your introductory paragraph, and it will guide the following body paragraphs.  I like recommending a two-part thesis statement because it usually allows writers to make a point and counter-point argument.  In your situation, the two-part thesis statement will still work because it allows the thesis statement to be stated in a cause and effect format.  I would start the thesis with the word "although" because it forces a dependent clause.  That, in turn, forces a following independent clause.  For example:

Although there are many different contributing causes to current obesity rates within the United States, the resulting health issues are all extremely serious.  

The above thesis is specific in topic.  It clearly states the essay is going to be about obesity causes and obesity effects.  It's also broad enough to allow you to discuss multiple causes and multiple effects.  
I also want to emphasize the need for a good first sentence.  I call it the "attention-getter."  The very first sentence of any essay or paper needs to grab reader attention right away.  A reader that is bored by the end of the first sentence isn't likely to keep reading.  I recommend one of four types of attention-getters.  
Ask a question.  A question immediately focuses a reader by requiring them to mentally engage with the question.  They are thinking about possible answers as they continue reading your paper.  
Use a quote.  By using a quote you draw special attention to the words in the quote.  Readers assume that the quote is important and noteworthy; therefore, it should be paid closer attention.  
Use a bold, perhaps controversial statement.  The goal is to get an emotional response out of your reader.  Emotionally engaged readers are readers that typically want to keep reading.  
Use a definition.  It's effective, but it is my least favorite of the attention getters.  In the case of your essay, I would simply provide a definition for "obesity."
Since your essay needs to be about the causes and effects of obesity, the body paragraphs need to discuss possible causes of obesity.  Your essay is only five paragraphs; however, obesity is likely caused by more than five factors.   Pick one or two that you feel are most prevalent or that you feel most passionately about.  You might even be able to combine multiple contributing factors together.  In my health class, we spend a lot of time talking about nutrition and exercise.  I don't want to say that the only causes of obesity are poor diet/nutrition and lack of exercise, but I do think those are major contributors.  You could definitely fill a paragraph or two on those two topics alone.  
As for the effects of obesity, you can't simply tell your readers that obesity is unhealthy.  You need to give specifics.  Obesity can result in diabetes, high blood pressure, increased cholesterol levels, increased risk of heart disease, and extra wear and tear on bones and joints.  Those are only some of the physical health risks associated with obesity.  Obesity also carries emotional, mental, and social health risks too.  You might want to include some of those as well.  
Close out your essay with a final concluding paragraph.  Don't forget about this paragraph.  It's a key paragraph because it summarizes the previous 3–4 paragraphs and calls attention to the initial thesis again.  Lastly, a strong conclusion needs to push readers toward caring about your topic and even push the reader toward taking action.  This is the one place in the paper where your "voice" and opinion can come through quite strongly.