It is impossible to exaggerate the importance of this step. While requirements will always change, doing a solid job now will set a good foundation for later steps. Conversely, groups that fail typically have only a general idea of what they want to do by the time they finish this step, a problem that seems to never go away.
Note that you do not have to worry about the customer changing his or her mind. Even if one of your team members has a big idea a week before your presentation, a simple "let's get something to work for the presentation" can be quite convincing.
To try to give you some feel for what industry thinks is good for requirements document, the requirements document this quarter will be based on the sample documents that come with Rational RequisitePro, the requirements tool within Rational Enterprise.
To find the sample documents on the machines in the PC lab, go to the Start menu in Windows and click on Start->Courseware->Computer Science ->Rational Software->Rational Requisite Pro. Click on the Learning Project - Traditional or Learning Project - Use Cases to check out each approach. In each case, the documents can be found by clicking on the Word documents listed in the navigational frame. Rational's Unified Process is based on the use case approach, but I believe that the traditional approach will be easier for most people to handle. For convenience, you may certainly turn in a single document for your submission.
You are not required to use RequisitePro, and even if you want to use it, remember to consider the PC lab schedule. We will show various features, but certainly are not going to cover everything.