The process begins with you registering with companies that offer mystery shops. At the bottom of each page of the forum is a link to our list of companies that do that. Register with a few at the top of the list and fairly promptly you should be accepted as a shopper by one or more of them. You then can visit their job board to self-select shops or request shops. Start with only one. Read the instructions carefully, take any required tests and perform and report it on line. Follow all directions, including where and when to do the shop, what you are to do, what proofs of visit you need and any deadlines for reporting the shop.
More information is available about all this in the New Mystery Shopper section of this forum. Take a look especially at the topics pinned to the top of that section of the forum.