I suggest that you start by reading all of the topics in the New Mystery Shoppers area of this forum. Then, subscribe to Jacob's (free) Mystery Shopper Magazine (online) and read the back issues for tons of info on types of shops.
Strat registering with the Mystery Shopping Companies (MSC) listed at Official List of Mystery Shopping Companies, below.
You can search the forums for comments on almost every one of them. But the key thing is to start registering. No one, of 15-20 MSCs will give you much of a range of shops, so grow your new business by signing up with 50-100 companies over the first few weeks.
Many shopps advise greeting your feet wet with bank shops where you interact with a teller, or any fast food or casual dining shop that does not require split second timing, or visits to cell phone stores. Take just one shop of a kind to start, so that you can see if you are comfortable with the report and guidelines.
Enjoy, and keep asking as you go.
Welcome to the forum!
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel
Poor customer service? Don't get mad; get video.