Qualifications
Can you mop a floor? Congratulations, you're hired.
Unless you're working with hazardous waste, which requires some training and a certificate, you don't even need a high school diploma to land this gig. However, if you want to stand out from the rest of the applicant pool, you can get some professional training through the following organizations: Building Service Contractors Association International, the IEHA, and the ISSA, The Worldwide Cleaning Industry Association (source).
Beyond an ability to clean things, and possibly a certification, you'll need to be reasonably physically fit, cool with working long hours by yourself, able to stay calm in the face of disaster, and not be too squeamish. We don't want to gross you out or anything, but janitors have to deal with some pretty nasty messes from time to time.