NEVER, NEVER, NEVER pay anybody to get into mystery shopping! If they want YOU to pay THEM, odds are you will get ripped off. A "legitimate" ms company should pay YOU.
You can register using the SAME list of legitimate companies that they will charge you for ABSOLUTELY FREE (see ms company list here, at volition.com, walletboosters.com or several other places)!
Register with as many of them (there are some 500 or so out there) as you can and you will find some that have available shops in your area...and just because they don't have shops in your area right now, it doesn't mean that they won't have a shop later...and you'll already be in their database as an potoential shopper.
It's true that you MAY be required to "front" a few dollars ($1-$10 for
most "new" shoppers) to fulfill the shop requirements and/or in order to then use the receipt as "proof" that you were there and the time of your shop,but MOST ms Companies will reimburse you in your shop fee payment. How much you have to "front" depends on the type of shops you take, how many shops you take and what they require you (if anything) to buy.
When you first start out, put ALL the payments into a "separate" mystery shopping fund and use THIS money to meet any future purchase requirements. This way, you're using ms money to pay for ms purchases and, especially as the balance grows, it will take NO money out of your pocket to do the shops.
So, in essence, apply, apply, apply, read, read, read (these forums and any other information you can find about mystery shopping) and learn, learn, learn.