Hi, jeszmay, welcome to the forum. Being a mystery shopper means managing your own small business, working as an independent contractor to multiple mystery shopping companies. Starting a mystery shopping business is very time-intensive at the beginning, because you must read and research and start your business.
To find out all the information you need to start and run your mystery shopping business, go to the New Mystery Shopper area and read everything there. After you have a plan in mind, go to the blue link at the bottom of the page, Official List of Mystery Shopping Companies, and register with as many mystery shopping companies on their websites as you can. I suggest registering with 50-100 companies minimum for the best volume of shops in your area. When you register with a company, you will provide your personal information and your social security number. Some will ask for writing samples and others may want you to take tests. After you are accepted by a mystery shopping company, you will be able to search their job board to see if they have assignments in your area. You can select the types of assignments you want to perform and apply for those jobs.
Market Force is a good starter company. They are a large company with a lot of shops in most areas. They usually have very clear directions and their reports are fast and easy.
Good luck with your new business. Happy shopping!