Welcome to the forum! You will be fine. At the bottom of each page of the forum is the forum's "Official List of Mystery Shopping Companies". These are all legitimate companies. Like in any business, some are more pleasant to work with than others, and you will see links to discussions about them as you decide to start registering. To protect yourself from scams: 1) Legitimate companies do NOT send you money in advance to go do your shops. You select or request a job, go perform and submit it, get paid anywhere from 2 weeks to 3 months after your shop is accepted (depending on the pay schedule of the company you are working with. 2) Generally you need to find the companies to register with rather than them finding you. 3) Almost all the time you are registering on a secure website rather than sending your personal information as a response to an email.
When in doubt, ask! The old adage of, "If it sounds too good to be true, it probably is." will keep you safe.