Welcome to our enchanted kingdom of mystery shopping!
First of all you get signed up with every legitimate mystery shopping company out there, at least as many as humanly possilbe. Many of us are signed up with over a hundred if not many more than that. Sharpen your acting and observational skills because you'll need both, also a humble attitude of learning is important. I've seen too many newbies think they know everything based on the 20 shops they've done, not that you'd effect that attitude but I've seen it too many times.
MOST importantly, and heed my words, do not EVER EVER EVER EVER pay any company to let you work for them no matter how special they try to say they are or how they'll try to tell you that you'll see shops with them that you see with no one else or this bull crap about how you're "investing in yourself" by shoveling money in their direction. You're not out there to make her car payments for her and to fill their pockets, they're supposed to pay YOU to work, not vice versa. I've been out here doing this for many years, 15 to be exact, my husband was a politician once so my bull-crap sensor knows what it sees. Don't fall for whatever story they tell you.
Good luck!
Her Serene Majesty, Cettie - Goat Queen of Zoltar, Sublime Empress of Her Caprine Domain