Hello!
You have come to the right place to learning about mystery shopping! First, I suggest that you take a look at the "New Shoppers" area of this forum. There is a tremendous amount of information there that will really open up the world of mystery shopping to you. Thereafter, you should look at the huge list of mystery shopping companies linked at the bottom of this page - over 200 of them. Starting signing up. I suggest starting with the "15 Most Discussed" as they are all larger companies that have work nationwide.
Feel free to ask questions on this forum. There are quite a few experienced shoppers around that will answer questions, with a handful of exceptions (such as asking which client is shopped by which MSC).
Hard work builds character and homework is good for your soul.