Welcome Lanel!
I hope you'll find this forum a good source of information on Mystery Shopping (MSing) and a great source of entertainment. I'm assuming that you're new to MSing since you just found us. Let me encourage you to read through the first six posts from Flash in the New Mystery Shopper section first. This will answer many of your newbie questions and give you a great head start.
There is a link to the official list of mystery shopping companies at the bottom of this and every page. Don't bother asking the forum which ones are the best. The answer is "it depends" - on where you live, what you enjoy, etc.. You'll need to read the forum and apply with lots of companies to build your own list of best companies. You will find that there are a few (very few) companies that have had issues with shoppers getting paid, etc.. However, most of them are decent to work with.
I can suggest to you that Marketforce (MF) and Corporate Research International (CoRI) are two great companies to get started with. They have some pretty easy shops in most areas and are good about working with new shoppers.
Enjoy!
AndrewTX
Certifiable