Tips: Make sure the companies you sign up with are legitimate (that is not to say they ae good/bad/indifferent, but at least they are actually companies). There is a list here in the first post of the first thread of the Mystery Shopping Company Discussion section of the board.
You do not EVER need to pay to shop and if you are asked to do so, move on as it will do you little, if any, good.
Take your time with your applications as the demographics will qualify you or disqualify you from various types of jobs and most of the companies will not even consider doing business with you without a complete application, including SS#.
BEFORE YOU START SIGNING UP. Let me encourage you to open a free gmail account to handle all of your mystey shopping business exclusively. There have been issues with Yahoo and AOL over the years with some legitimate companies being detected by these providers as spammers and their communications deleted or returned to sender without ever getting to you--even to your junk mail box.
Set up a Paypal account because as you are applying, many companies ask if you will accept payment via Paypal. Some companies ONLY pay via Paypal. Use your new gmail address for your Paypal account.