One possible focus of your paper might deal with the famous scientific accomplishments of the Mayans and the famously violent religious practices of the Aztecs. This, at least, is how the two cultures tend to be remembered today -- the Mayans for their calendars and the Aztecs for their violent religious rituals.
Of course, before you can write your essay, you need to do some research, preferably using peer-reviewed sources (sources read and approved by experts). One danger of simply using the web for research is that often web sites are not peer-reviewed, or at least they are not obviously labeled as such. Sites sponsored by universities are often reliable; they usually end with an "edu" suffix. Sources found on Google Books tend to be more often peer reviewed than those found simply through a Goodle web search. Databases to which your school subscribes would probably be trustworthy; your school librarian can help you with those. And, of course, old-fashioned books and journal articles in the library can be helpful as well.
In the meantime, here are two sites that may help get your started:
You may also want to search the terms "Aztec" and "Mayan" in the eNotes search box in the upper right corner of this page.
Good luck with your project!