I have to echo Marc's reasoning here. With using the kit, you should be starting to have some inkling about what you most enjoy about photography. Landscapes, portraits, kids - grandchildren, architecture, sports, travel, and/or ?????? Looking through your past images will certainly help. So, based on what and how you use the camera, that will point you to the general area that a prime may be of most use to you.
You do not have a general use yet, and just want a prime? Then, you are following the whims of everyone else. Then, find something very inexpensive, to scratch the itch, an older M 50mm f2 for under $30 or a Tokina 28mm f2.8 for around $20 or under (either of which might just come with a older film body attached).
Even with a good idea of the focal length area that appeals to you most, then there is manual, automatic, autofocus, new glass, old glass. Regular lens or specialty (fish eye)? Pentax, or third party - Sigma, Tamron or some super premium like Zeiss...