Hi, samara, welcome to the forum! Being a mystery shopper means running your own small business, working as an independent contractor to multiple mystery shopping companies. Beginning to mystery shop is very time-consuming initially, because you will be required to read and learn. The time you spend will be an investment in your new business. All the information you need to start and run your own small mytsery shopping business is available free here on the forum. It's set up so you can read-as-you-go, learn-at your-own pace. A good place to start is in the New Mystery Shopper area.
You need to register with companies, providing your SS# and personal information. Some want you to take tests and you may be asked to provide writing samples. After being accepted to by a company, you will be able to log into their website and look for assignments in your area. Some companies allow you to self-assign the assignments. Other companies allow you to apply for each assignment and the scheduler notifies you if the assignment is assigned to you.
There is a lot of competition in most areas. My advice is to register with as many companies as possible - many of us here on the forum are registered with 150+ - and spend a lot of time reading and researching so you figure out how mystery shopping works and know how to set up your business. It's very time-consuming to get started. Don't get discouraged.
Good luck and happy shopping!