First, welcome to the forum. Second, there is a list of legitimate mystery shopping companies link at the bottom of each page. The link will also show you a link to discussions of the company.
Mystery shopping as we do it is contract work--each shop is contracted for individually--so 'earn an income every month' is dependent on the shops you accept, perform and report as per instructions. Companies will pay you anywhere between 2 weeks and 3 months after a successfully completed report, depending on their payment schedule. We do not encourage you to sign up with any outfit or service that charges you a fee, though most shops do require some sort of purchase in order to get a receipt and offer some sort of reimbursement of that expense.