Good Morning!
I have been doing this for a little over a year and the best "advice" I've gotten has come from this forum. Check in frequently and you'll get ideas on everything from how to handle odd situations, to what sorts of shops are available, to warnings about scams, to different personalities of MSC's and more.
I would start in two places: iSS (isecretshop.com) and presto instashops. iSS is a platform on which different companies post their jobs. You can sign up for these companies very quickly through the iSS webstite. Presto Instashop is a map of work offered by different MSC's on the "sassie" platform. For this one, when you locate a shop that interests you, you'll need to go to that MSC and register for it individually. You should then check their job board.
Thereafter, as you read things on the board that interest you, sign up for that company and see what's in your area. There are over 300 mystery shopping companies and not all will have work in your area, so the registration process can be frustrating. Just keep at it.
If you can think of a business, it is almost certainly shopped. In the last year, I have gotten my car serviced, eaten at high end restaurants, eaten at more moderate restaurants, opened bank accounts, run bar integrity audits, shopped new cars, shopped apartments, asked retailers about credit cards, had pizza delivered to my house, inquired about cell phones, inquired able cable TV services, gone to professional baseball games, attended rock concerts, mailed packages ..............
Some do this to augment their incomes. Some make a living doing this. I was able to bring in nearly $1000 my first month doing this - in addition to my full time job.
Good luck!
Hard work builds character and homework is good for your soul.