Use the adverb atop when something is on the top of something else. You could say, for example, "I'm standing atop of the chicken coop, getting ready to jump in that pile of leaves."
The word atop is somewhat old fashioned, but it's still a good way to describe being at the very top of something. You might live atop of the highest hill in town, or enjoy looking out your window to watch birds land atop of the garage. In the seventeenth and eighteenth centuries, the word was spelled with a hyphen: a-top. It took its current form, atop, in the nineteenth century.