Hello and Welcome!
First, as a new shopper, I suggest that you go to the "New Shopper" area on this forum as it contains a tremendous amount of information and will really give you an overview of the world of mystery shopping. Of course, if you have questions, feel free to ask here, you will almost certainly get multiple answers. (It's a little slow right now - in part due to the fact that it's the end of the month and a lot of shoppers are busy chasing down end-of-month bonuses.)
As for your question... There are over 200 mystery shopping companies, some large with work available nationwide and some smaller that might only have work in various regions. At the bottom of this page, there is a link to a list of mystery shopping companies (MSC's). If you click on the name, you'll get a list of discussions about that company and a link to a page on which you can sign up for it. I suggest starting up with the fifteen most discussed companies as they are all larger companies with work pretty much everywhere - in other words, they are the most likely to have work for you. Over time, if you decide this is for you, you will want to sign up for most, if not all, of them.
Hard work builds character and homework is good for your soul.