Hi, welcome to the forum. Getting started is a lot of work and very time-consuming, so set aside some time to invest in starting and growing your new mystery shopping business.
The blue link at the bottom of the page, Official List of Mystery Shopping Companies, is a list of reliable, legitimate mystery shopping companies you can sign up with. You will go to each company's website and complete an application, giving your personal information and SS#. You will be asked to sign an Independent Contractor (IC) Agreement, including a confidentiality agreement. Some companies require that you submit a writing sample or take tests. After you are accepted as a shopper by that company, you will be able to search their job boards to see if they have jobs in your area.
Any question you might have about how to start and run your business is answered in the New Mystery Shopper area. I suggest you take a bunch of time and read everything in the top 6 threads in that area. Then read as much as you can everywhere in the forum, because good information has been discussed.
You can always ask questions, but before asking a question about a company or pay method, use the "Search" feature, selecting "all dates" to search and see what has already been written. This avoids asking a question that has already been asked and answered. The "search" button is above on the blue line directly above the lists of forum topics.