Read as many forum posts here, and at Volition as you can. Apply for as many companies (the ones with the good reps only, and NO pay to shop companies!) as you can.
There is a list on this site, and one on Volition with hundreds of mystery shop companies.
Some people recommend Shadow Shopper but most of us with experience don't believe in paying to find shops. Some will disagree, but, that's a decision only you can make.
Basically, it's read read read, apply (after checking the rep), apply apply...
Learn what is valued in a shopper: Reliability, integrity, objectivity, observational skills, and being able to express all that you have seen and heard.
Good luck!
~Christina