Read the posts on the New Mystery Shoppers section. The ones pinned at the top will give you plenty of information.
Then sign up with many Mystery Shopping Companies (MSCs). There is a list here. Not all of them will have shops in your area. Most people have signed up with over 100 of them but work consistently for less than 10. But you never know when a shop will suddenly appear in your area from a MSC that you don't hear from often.
Read and follow the guidelines for each shop.
If you sign up for a shop, complete the shop so you don't get a "flake citation". These can hurt your ability to get future shops from the MSC.
I'm sure others will post more information for you.