I suggest you write about Egyptians and their mummification-preservation of the dead system. Not only we know a lot about their culture, but also they have a set of their own unique gods, customs, and history. Also, honestly.. I can't even think of another ancient civilization that had such an interesting death system.. others may have done stuff too, but it's not as elaborate and there's really not much to write about... There's also lots of movies on the subject, so you wouldn't be left confused. Ancient Egypt also had their own writing system with their own hieroglyphics, and not every ancient culture can brag about that. Sure, the Mesopotamians had their cities and writing, but from a student point of view, Ancient Egypt is a lot easier to write about since it's a unified culture that lasted a long period of time with many events going on. If you were you, I'd go with the Egyptians.