Look at a specific advertisement to see how it works and see how it tries to sell the product/service it is advertising.
You could research some background information on the company that is advertising the product in your advertisement. Or, you could research some information on the person or product who is present in your advertisement. Brainstorm and begin writing a draft that uses textual support from the advertisement (discussion of the rhetorical appeals logos, ethos, pathos, images, color, words, hidden messages, etc.) and textual support from the related source you found.