Greetings!
Well, the first thing is to make sure you understand the differences between the two--classical Athenian and modern democracies. As you say, you'll need to know the key features of each. You might be surprised at how much information you can find just by doing an internet search with the terms "key features Athenian democracy." For example, I discovered that the chief difference in the Athenian model was that the people voted directly on laws and bill themselves, without electing representatives to do it for them. And of course, not every member of society was eligible to vote, an important distinction as well.
Do your research and start by making lists; that will make the actual writing easier. Don't worry too much about how to make it long enough. By the time you have explained the key features of each system, you'll be well on your way. Then, you can fill out the rest by answering the question which asks for your opinion on western democracies. Giving your opinion is always good for expanding word count! :-)
If you'd like to post your rough draft when you have one, I'd be happy to help with some editing!
Thanks,
Sarah, EssayForum.com
Sarah, EssayForum.com