Actually you have already pointed yourself in the right direction. This forum has a wealth of information at your fingertips.
At the bottom of the page is an "Official List of Mystery Shopping Companies." All of the companies are legitimate. It still doesn't mean they are all created equally. There is a link to discussion about the company next to each one's name. In addition there are a couple of sections such as the are for New Mystery Shoppers, that have "sticky notes" at the beginning. There you will find information on anything from getting started to equipment to advice on taxes. Settle in and start reading and you will be forewarned and forearmed
After you've read for a while and even started shopping, feel free to ask questions, the more specific the better as you will get responses from people who have probably had the same experience!
Equal rights for others does not mean fewer rights for you. It's not pie.
"I prefer someone who burns the flag and then wraps themselves up in the Constitution over someone who burns the Constitution and then wraps themselves up in the flag." -Molly Ivins
Never try to teach a pig to sing. It's a waste of your time and it really annoys the pig.