First of all, do not expect to go into this earning a "salary". We are Independent Contractors. We go to the job boards of companies we are signed up with and request jobs we are willing to perform. Sometimes we can assign the work to ourselves directly and other times we can only request, with the company's schedulers deciding to whom to give the job.
How much you can make depends on the jobs available in your shopping area, how diligent you are in seeking out work to request, your reputation with the companies with whom you have signed up for being a reliable and thorough shopper, etc. etc.
"Good" company/"bad" company is a matter of opinion and where you are both in your career as a shopper and your physical location. There are some companies that really won't bother messing with you until you have shopped for several years. There are some companies that focus on working with new shoppers and offer instructions and training that are rather tediously complete.
What you are doing when you start mystery shopping is running your own small business. Unlike an employee situation, there are no sick days or vacation days or 9 to 5 days, etc. Nobody can tell you what the "good" companies are for your area. In fact, nobody can even tell you which of the 200 or so legitimate companies on the list in the first post of the first thread of the Mystery Shopping Company Discussion portion of this forum will have work in your area. That is something you will need to explore as you grow your business. What we can tell you with a fair degree of certainty as that Bare, Market Force (blue portal), Corporate Research International and Maritz are very very likely to have shops in your area. These companies have a good reputation for paying shoppers in a timely fashion and for being perhaps more lenient than other companies in accepting shops with slight shopper boo boos. At the same time, they have so many shoppers available that they can afford to pay lower fees than many companies out there. Get some experience, get signed up with lots of companies, do your best on every job you do and over time you will find it easier to find work and be offered work that pays better than beginner shops. Realize, however, that the pay never gets really good--you earn every penny of it--but rather you will get faster and more efficient.