Welcome to the forum! Most of our mystery shops are not on line but rather are visits to physical locations.
You are wise to be cautious. If you remember that companies will NOT send you checks or put money on your credit card before you do a shop, you should avoid most scams.
The New Mystery Shopper area of the forum was set up to give folks the basics and at the bottom of each page of the forum is "The Official List of Mystery Shopping Companies" which will provide you with links to forum discussions about the companies as well as links to register with the companies.