Read, read, read everything on the forum. That's how I got started. Here's a list of companies to sign up with:
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www.mysteryshopforum.com]
Remember, when you are mystery shopping, you are not an employee of any company. You are an independent contractor running your own small business and the Mystery Shopping Companies are your customers.. Because you are running your own business, you need to spend time to learn how to do it (reading here will tell you pretty much all you need to know), researching and signing up with companies. Good luck with your new business and have fun!