Your research methods and aims are no different here from those you use in preparing a term paper. You want to try to master the ideas in your subject firmly. You want to bring your own critical thinking skills to bear upon them. You have the added pressure, however, of public exposure of your preparation. Your classmates will be able to tell if you are just regurgitating half-understood ideas you've read in your sources. They will be much more engaged with your material (and much more impressed by your seminar) if you bring something of yourself to your topic. Do you agree with what you've read? Do you have your own take on the material? Don't be afraid to air your views -- and be prepared to defend them! This is a central challenge -- and a good deal of the fun -- of being a scholar.
If you are preparing a joint presentation with other students, you need to decide early on the division of the topic among yourselves. You should make such a division with an eye to your eventual presentation. You will want to divide responsibilities for presentation so that the seminar hangs together and flows smoothly, so that you're not uncertain about who is to do what, and so that you're not getting in each other's way.
Finally, if you can, prepare your classmates also. Your seminar will always go much better if you deliver it to those who already know something about the topic. If the course setup allows for it, place some preparatory materials for them on course reserve a week before your seminar. Preparing for your seminar should be part of their course requirements! Explain clearly in your preparation material what the topic of your upcoming seminar is and what approach you intend to take to this topic. Suggest some readings that will introduce them to your topic.