I'd be happy to give you some general guidelines for writing a classification essay. :-)
Remember that the purpose of a classification essay is basically to organize things into categories so that they can be distinguished from each other. Therefore, start by thinking of the types of categories your "thing" (I gather it's advertising, here) can be sorted into. For instance, types of advertising would include print media, television, radio; or, you could go with the contents of the ads themselves: humorous, informational, trendy, traditional, youth-oriented (that brings up a whole other category, i.e., market-specific). Once you have decided what types of categories you want to use, make sure that you keep them organized by their unifying principle; that is, if you are sorting them into types of media (print, T.V., internet, radio, etc.) do not throw in a completely different type of category (e.g., retiree-oriented). Give several examples that fit into each classification.
I hope this helps you get started!