Best of luck with your new mystery shopping business. Be sure to read everything in the New Mystery Shopper area so you will know how to start your business. To find assignments, see the list of legitimate companies below, the blue link at the bottom of the page, Official List of Mystery Shopping Companies, and start to sign up with companies. When you register with the companies, you will provide your personal information and SS#. Many will request that you provide a writing sample so they can see that you are able to write acceptably and that your writing uses correct spelling and grammar. Some will request that you take tests. Once you are accepted by a company, you will be able to visit their job board to see if they have jobs in your area. Most successful mystery shoppers are signed up with 150-200 companies, so I would suggest you sign up with at least 50 to give you a good variety of assignments in your area.
There is a lot of competition for jobs, so many are taken as soon as they are posted on the job boards. Jobs are also released at different times during the month, so be sure to check back frequently. Many companies give you the option of receiving an e-mail when there are shops available in your area.
Good luck!