Welcome to the forum, Fay... glad you're here! I recommend signing up with lots and lots of Mystery Shopping Companies (MSCs) so that you have plenty of shops to choose from when arranging your schedule. There is a list at the bottom of the page called "Official List of Mystery Shopping Companies" to get you started. What I have done is research each company BEFORE I sign up with it to make sure they pay their shoppers in a timely manner and have a good reputation in other ways. You'll find that some shoppers don't mind waiting 90 days or more to get paid, while others won't work with companies like that. We each have to make our own decisions about what works for us when it comes to getting paid.
Keep coming, Fay, and let us know if there's anything you need to know as you're getting started with your new career!