Hey RunsWithScissors,
First and foremost find a topic you are passionate about. If you don't you will not enjoy the process one bit.
Secondly find something that has a large scope.
Three try and go against the grain a bit and go in depth on something that's not going to be overdone by the state aka holocaust etc. (No offence to those who did it, you can certainly still pull something amazing, but it's just so overdone and obvious).
Four - Get it done in Xmas holidays (I was warned by the year before me) I wish I had heeded their instructions. I finished on time but I certainly hated leaving it to near the end.
Five - It's worth 80% of your internal mark, pour your heart into this. Don't stress with it but follow a clear and concise structure and just go crazy on historiography, various factors and the interplay of differing schools of thought.
The topic I did is lengthy to explain, but my title should make it obvious.
“The Age of Enlightenment or the Fall of Inquiry: A study into the fall of the Middle Ages and how misconceptions shaped History’s supposed ‘Golden Age of Thinking”
Edit: Books, books and books. Yes you can have plenty of online sources, but book are your biggest friend. Go out and get some that cover your topic really indepth. I was lucky to find 2 books, each with a historian arguing opposite sides of my argument. Books like these will make your argument 1000x stronger, you'll be better read and your teacher will definitely appreciate the fact you put some effort into your research.