Hello and welcome, Peaceangel!
Rule #1 of Mystery Shopping is never pay money to shop, and never cash an unsolicited check from a company offering to send you funds up front. That's the biggest scam out there.
Rule #2 is that Mystery Shopping is a versatile business, and is different for everyone, so you may have to dabble in a wide range of areas to zero in on what is best for you. I like doing apartments, bar integrity, and nice restaurants, and usually steer clear of banks and gas stations. Many people are the exact opposite.
"The future ain't what it used to be." --Yogi Berra