Hi, Fantastic_1, welcome to the forum. Check out the New Mystery Shoppers area and read as much as you can here on the forum. Being a mystery shopper means running your own small business working as a independent contractor for multiple mystery shopping companies.
All the information you need to set up and begin your mystery shopping busines is here. It's set up as a learn-as-you-go, work-at-your-own pace process. The startup is very time-consuming. You will spend a lot of time reading so you can understand how mystery shopping works and researching and planning and setting up your business. Then you will register with companies (see the list below!), providing your personal information and SS#. Some of the companies will want you to provide a writing sample or take tests.
Very time-intensive. Don't get discouraged. Plan to spend a lot of time in preparation before you start making any money. You'll spend a lot of time sorting through jobs to decide which ones are "worth it" to you and finding companies that have assignments in your area. When you do start getting assignments, most will not be quick, easy, fast, or fun. And most will not pay very much money. In most areas, there is a lot of competition.