How can I write a five-hundred-word essay describing a bad situation or problem in my community that has had an impact on me personally?

Only you know about a bad situation or problem in your community because it is your community.  But I can offer some possibilities and give you some help on how to write your essay. 
Two particularly bad problems in communities are drugs and crime.  Two others that come to mind are unemployment and poverty.  Still another is the homeless, who seem to be in so many communities today.  Environmental damage might be a problem in your neighborhood, too.  These are serious problems, and your assignment sounds as though you are meant to address a fairly serious problem, not a problem such as litter or not enough trees in the park.  Do any of these problems exist in your community?  You could choose one of these, or use the list to get you thinking about some other problems you have noticed. 
As far as I can tell, this assignment seems to be a kind of hybrid of a problem/solution essay and a cause and effect essay.  You are expected to state the problem and discuss its causes.  I do think, though, that in order to give your writing some purpose, it might be good to focus at least a bit on a solution or solutions. You might do that as you discuss the causes, paragraph by paragraph, explaining to the reader how to address each of them. 
Now, your introduction needs to explain the problem and also include a thesis sentence.  Your thesis sentence can be a cause and effect kind of statement, naming the effect, the problem, and the causes of the problem, which will be your supporting points.  Let me give you an example of what I mean.  If I were writing an essay about a problem in my community--the homeless people I see from time to time--I could have a thesis statement like this:

The problem of homelessness in my neighborhood needs to be addressed by attacking its causes: poverty, mental illness, and a shortage of housing.  

I can end my introduction with that thesis statement, and that helps me to structure my body paragraphs, the first on how to address the poverty, the second on how to address the mental illness, and the third on how to address the housing shortage.  Notice that my body paragraphs will be in the exact same order in which they appeared in the thesis statement.
For each of these body paragraphs, you need to write a topic sentence that lets the reader know which cause you are focusing on. Then you can go on to develop the paragraph with evidence to show the causes (and a possible solution) to your reader.  The evidence might be some statistics or it could even be anecdotal, but you must provide some evidence of some kind to make the point you want to make.
Finally, you will need to write a conclusion in a separate paragraph. In a conclusion, you should remind your reader what the problem is, what the causes are, and possibly exhort your reader to help solve the problem.
It is good to take an interest in the problems of one's community. If we don't, we are not acting as good members of society.