Hi!
First, do some reading. There is a "New Shoppers" area in this forum with a ton of information for you - it will really give you a good overview of mystery shopping. Second, you will need to sign up for legitimate companies. There is a huge list linked at the bottom of this page. I suggest starting with the "15 most discussed" as they are all larger companies with work nation-wide.
There is a huge amount of information shared daily on this forum. Follow conversations. Ask questions. It's not hard to get started and this is a great gig for somebody who needs flexibility.
Hard work builds character and homework is good for your soul.