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.