This seems like a potentially controversial topic, so my first recommendation is to consider your reader. If you truly don't have an opinion either way on the prompt, try and think about what opinion your reader is likely to have. If your essay supports what your reader already believes, then your reader is a supportive audience member and is less likely to find fault with your opinions and is likely going to be more forgiving with things like grammar and usage mistakes.
Regardless of your true feelings on the issue, your essay is going to need a thesis statement. The prompt is essentially giving you an "either/or" setup, so my recommendation is to use a thesis statement that allows you to discuss both sides of the issue. This will help you out if you know that your argument is counter to your reader's argument. This is because a significant portion of your essay is going to discuss the opposing argument(s). Then the paper will engage with your argument and explain why it is superior to the alternative argument. This kind of thesis statement is usually a complex sentence that begins with a dependent clause. For example,
Although the mainstream media supports the idea that it benefits American democracy, the truth is that the media does far more harm to democracy than good.
That particular thesis statement is easily flipped around to support the opposite main argument.
I don't know what your feelings are, but I believe that the essay should definitely explore the First Amendment as it protects liberties like freedom of speech, religion, press, petition, and assembly. The press is effective at getting information out to the general voting public. That is a good thing for a democracy; however, the media can also be guilty of not giving people all of the necessary information. This could be because of time constraints or because producers and editors decide what information will be given to the public. This builds into the media an unavoidable bias, and that could be where you engage the argument that the mainstream media undermines a democracy.