NEVER, NEVER, NEVER pay anyone to do mystery shopping...the idea is for them to pay you!
There is a list of "real" companies that you can register with (all free) here as well as at volition.com, walletboosters.com and others. Register with as many companies as you can (start out by registering with just 3-5 a day during your first month and soon you will have a list of companies that you are registered with that surpasses that of most newcomers...and some long-timers too! There are some 400-500 companies or more that do mystery shopping, so registering with new ones (like learning about all the aspects of mystery shopping) can be an ongoing thing.
Keep an alphabetical list of of all the companies along with your shopper number for each and any "unique" login information (username, password, etc.). In most cases, you will be directed to their login page. I have found it helpful to create a MS Company folder in my "Favorites" and add this link for each company as it may be different than the company's name.
Be sure to create a separate email address for your mystery shopping emails and an account with PayPal that uses THAT address before you register with anyone, as you will need this email address and account information to register.
In a day or two, you SHOULD be on your way.
If you need help, just ask...SOMEBODY is bound to know the answer.