First it is always best to focus on topics on your subject of most interest to yourself, but within the parameters of the assignment. What questions do you, yourself have about the subject? Exploring such questions will help you focus and produce a more interesting research project. It is important to see what sources are available for a particular topic to help decide between topics.
The less you know about the subject to begin with the harder it will be to choose a topic. If you have no idea where to begin, some background reading will help. You might find doing some initial reading on a subject using a general source like an encyclopedia such as wikipedia might help gain some background in order to pick a topic, but don't rely too heavily on such a source for the actual research portion of your project. It is a good place to begin, but don't limit yourself to such a source, and remember to check facts across a number of sources especially when using the internet. Also find sources near your reading level, cutting and pasting isn't research and you will not gain an understanding of your topic if you cannot understand what you are reading. Never quote or paraphrase information you do not comprehend, it will produce low quality work.