Cettie has given you some great advice!
I wanted to chime in. As a mother to 2 young kids, there are many assignments where you are not allowed to bring children. This has resulted in either having to find a babysitter (if husband is busy) or turning down assignments. That's something to keep in mind.
Try not to book too many assignments in the beginning. It may overwhelm you, at least that was my experience. The shops don't normally take long-- it's entering in all the reports that can be a chore.
Like Cettie said, it can be feast or famine. Some months I don't have many jobs and then wham, towards the end of the month I'll have a lot of companies begging me to do jobs.
I certainly don't make tons of money. Many of the jobs require you to make a purchase and you are reimbursed up to a certain amount. Example, K-Mart requires you to make a purchase, but you're only reimbursed for up to $1. McDonald's requires you to purchase a meal in their drive-thru and dining room (and you can't bring anyone with you), but you don't get reimbursed until several weeks later. Some companies are very slow to pay, taking up to 6 months or sometimes even longer! So, be prepared for the fact that you will be spending your money initially on meals and purchases upfront before you get reimbursed.
I'm not trying to scare you off
I love mystery shopping, most of the time. It can be challenging but it can also be interesting and fun too.
Good luck!