My tip to you is that you get a good book on Greek mythology (or Sumerian, Norse, Chinese, whatever - Greek mythology isn't the only one available, of course) or find a good website on the subject. Any 4 year old can do a Google search or ask a librarian for a book so that shouldn't be a problem.
Then, just read the story of Odysseus (if that's the one you'd like to do your talk about), read it a few times, and you'll get engrossed in it and there will be no need to force it into your head since it's always easy to re-tell a story by heart if you like it. That's how I got into Greek Mythology at age 11 - I too had to do a presentation in class on a topic of my choice, and ended up spending most of the day talking about all the myths and getting an A+ for the presentation. You just have to do it by yourself, not expect others to be able to do it for you.