Housekeeping has been a valid profession for many men and women for hundreds of years. In the Victorian era there were schools available that trained prospective maids and butlers in the art of cleaning silver, polishing marble floors, and cleaning expensive silk dresses without a dry cleaning machine. Etiquette, grace, and a good work ethic were also required of these specialized workers.
In modern times it's a little easier to become a housekeeper, and it doesn't have to be a life long career. While most men and women know the basics of cleaning, many people have never polished silver or done many of the specialized cleaning a housekeeper may be required to do. It's also important to know which cleaners can damage materials, which tools work best, and how to get rid of stubborn stains and spills.
While the work may be easier for modern housekeepers, there is no longer any specialized schools or training programs available to train them. Most housekeepers are either self taught, or they are trained on the job. If you are knowledgeable on the basics of cleaning but would like to learn more and gain experience, it may be worthwhile to take a job for a hotel or cleaning company. Even if you intend to work for yourself in the long run, the job experience and teaching offered by working for an established company is hard to beat.
If working in a hotel or for a cleaning company doesn't appeal to you, you will have to learn what you need on your own. Any library should have at least a few guides available on cleaning techniques, especially stain removal and for specialized materials like upholstery. Also, practice on your own items and in your own home. It's a lot easier to deal with a botched cleaning job on your own sheets than having to replace something that belongs to a client.