Use both your extensive primary market research ("bottom-up") as well as "top-down" sources—the Internet, trade magazines, trade shows, Dewey library resources, etc. to figure out what the current market is for your product/service, and what options they have with regard to your solution.
Even if there is no existing market for your product/service, talk about how your target customer is currently spending his/her money, time and resources, and how you intend to take an increasing slice of that money, time and resources. Remember: There are always competitors, even for seemingly innovative products. And many times for customers, an appealing alternative to your solution is the option of doing nothing.
For the section on market segmentation, you might want to answer questions like:
At the end of this section, you should now have a logical flow from your target customer profile and persona to a highly credible initial TAM and then a larger TAM for markets that you may address later. You will also generally have yourself positioned as the most attractive solution and this will be solidified in the Competitive Advantage Section which will follow shortly. Again, it is important that you do all the steps on schedule and then refine further over the remainder of the semester.