Each side can be made up of one or more persons. I recommend that each member of a side actually give a speech, but it may be possible to have a group of people doing research on a side, and select one or two of their members to do the speaking. The research and preparation are at least as important as the actual presentations. If the proposition is a factual one, the prima facie requirement devolves into your profession's standards for proof of facts. In one field, this might require reports of scientific research, in another testimony, and in yet another reading from a literary text.
Policy debates are the most complex.
The negative's responsibility is to refute the affirmative. This may be done in two general ways. First, the negative may defend the status quo. This may be done by showing that the affirmative is wrong on any point necessary to sustain its case. Second, the negative may propose an alternative point of view.