Hello, viv. This forum, is full of mystery shoppers - new and old, exchanging ideas and asking and sharing opinions.
Landing shops is not done within this forum. Each mystery shopping provider/company has its own site, with its own clients. You will need to register with the MSP and then search their job board. If something on the board appeals to you, you apply for the shop, then wait to see if the MSP assigns it to you.
There's a substantial list of MSPs on this forum under the section titled Mystery Shopping Company Discussion. It's an intimidating list by virtue of the numbers. I would suggest you start with applying with MSPs who have a nationwide presence. After completing a few shops, if you find you want to pursue mystery shopping, return to the list here and register with as many as possible.
MSPs with a national presence are MarketForce Information, Bare International and Corporate Research International. You may also find out which MSP has shops in your neck of the woods by registering with JobSlinger. They have a free site to search shops by radius.
Good luck going forward. Come back with any questions or to share your shop stories.