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.
Like mentioned above the new shoppers area of the forum is great.. It really was informative when I was new. Is there anything specific you need help with or a question? If you can explain we can try to help..