Welcome Sandra. Go to the New Mystery Shopper section of the forum. The "Nuts and Bolts" thread towards the top of that section gives you some notion of how this all works. There are 3 other threads also at the top of that section that go into other aspects as well.
Mystery shopping is not a scam that requires you to purchase a lot of stuff you only marginally can use on line or in person. Often you will be reimbursed for a small purchase of $1 to $5 for a purchase made primarily to obtain a receipt showing where and when you shopped that is scanned and submitted to the company you did the job for as a 'proof of visit'.
There is a wide variety of jobs to be done. Some may require no purchase at all but getting a business card or other 'proof of visit'. Some may require a small purchase. Some may require larger purchases which you are to return so you can evaluate both the purchase and the return behaviors. Some may require larger purchases that you can return if you want to. All of these pay fees. There are some shops that reimburse your purchase but pay no fee. So basically there is a little of everything out there and you just have to read through the information for a particular shop before you sign up for it.