MANY shopper do this as a full-time thing...but, you have to WORK at it full-time (and, maybe, then some) if you want to make "full-time" money. Depending on where you live and the number of shops available in your area, this might not always be possible. I know several people who have made this into a full-time profession.
MOST shoppers, however, only do this part-time to make "extra" money. It's pretty easy to make $200-$500 a month (or more depending on how hard you work at it) doing this part-time.
If you're interested, take a look at the "How to get started as a mystery shopper" sticky/topic on the top of the this forum directory. There are posts there that SHOULD help you to get started and answer most of your initial questions. As you have other comments or questions, please post them. We have SEVERAL experienced shoppers as members of the forum and SOMEONE will, no doubt, be able to guide you.