I'm not going to do your homework for you, but I can make a general suggestion.

It sounds like you're reaching for random similar / different facts. I'd suggest taking a step back and thinking about bigger categories that can help guide your research and help you out together an organized diagram. You might look at religion: what did religion look like in the Ottoman Empire? How about for the Aztecs? You'll start to see similarities and differences pretty quickly. What about armies? Leadership? The role of women in society? Diet / agriculture / common industries? All these things will be easy to research if you get a few general books on the topic, or even just read through an encyclopedia.

Once you choose a few categories on which to base your comparisons, finding the details to make the diagram will just take a few hours of reading.

Good luck!