Welcome to the forum, jaredsua. Mystery shopping means running your own small business working as an independent contractor for mystery shopping companies and performing assignments with their clients. Getting started is very time-intensive because you need to read and research to understand how best to get your business running. It takes a lot of time to register with mystery shopping companies. I recommend 50-100 companies to give you a good variety of jobs in your area. When registering with each company on the company's website, you will provide your personal information and SS#. They may ask you to take tests or provide a writing sample. You will sign an Independent Contractor Agreement (ICA), which will probably include a confidentiality agreement. After you have registered and been accepted by a company as a contractor, you will be able to search their job board and apply for assignments in your area.
And all this before you do any actual assignments or get paid! Read as much as you can here on the forum. There is great information to help you get started. And a great list of companies to sign up with is on the blue link below, Official List of Mystery Shopping Companies.
Good luck and happy shopping!