Read up on metrosexuals, the definition of the term, other writers' thoughts on metrosexuals, and anything else that interests you. "Metrosexual" is a fairly broad term, and you need to refine your topic.
Write an argument about metrosexuals. This should be your opinion -- in order to make the essay interesting to write -- and it should be debatable. So, "metrosexuals are in touch with their feminine side" is not a very good argument, because it's just a fact -- it can be backed up quickly by simply referring to the definition of the word. However, "metrosexual men are changing male culture for the better" is a much more effective argument because it's not necessarily true -- and it needs to be backed-up with logical facts and explanations.
Write an outline for your essay. This should be a bullet-pointed list of ideas that will translate into your essay. Each heading should directly address the argument, and each heading should have at least two subheadings, with both an explanation and an example. So, your first heading could be "metrosexual men are making it more acceptable to be less stereotypically 'masculine'," and the subheadings could explain what this means and underscore it with a reference, personal anecdote, or another kind of example.
Translate your outline into a rough draft. Each of the headings should be the first sentence of a paragraph, while each subheading should turn into the sentences following it.
Edit your draft, then make any necessary rewrites.
Turn your final draft in.