Unless you are already an expert on the topic, do some research in the area of diversity and see what sparks your interest. Once you have determined a specific area of diversity you wish to focus on, determine the purpose of your essay. Will your essay be informative or persuasive? An essay can also be written to entertain, but that would be an unlikely purpose for an essay on diversity.
Once you have determined your topic and purpose, consider your audience. Who will be the reader of this essay? This will help guide your tone and language choice.
The next step is to write your topic or thesis statement, and then use your topic statement to create an outline that emphasizes the key points you wish to discuss in your essay about diversity.
Using your outline, complete a rough draft, using credible sources as references. Review your essay for cohesiveness, organization, and word choice, as well as grammar/spelling/punctuation. You might wish to have a peer review your essay as well and provide feedback.
Lastly, complete your final draft.
The first major step you need to do is narrow your topic to a specific type of diversity, as you will not be able to cover every single aspect of diversity in a single essay. You could address, for example, diversity in religion, economic background, regional diversity, ethnic diversity, disability (i.e. that true diversity must include both disabled and non-disabled people), etc. Next, you need to take a stance, or make a claim. For example, you could argue that a certain type of diversity is beneficial for some specific reason or you could argue that diversity is bad for certain reasons. The main thing you need to do is have a focused claim which is well supported with factual evidence.