HI!
I've been doing this a little over a year and feel like I'm doing pretty darn well. It's a little daunting at first, but after a few shops, you get pretty comfortable. Start by reading the information on this forum for new mystery shoppers. It's pretty extensive.
Then, pick a couple of places to get signed up. I suggest iSecretShop as its a platform with quite a few companies. It will have a variety of shops with (mostly) pretty simple reports. You fill out one application and are suddenly enrolled with twenty or thirty companies.
Pick one or two shops that are places you would be comfortable as a customer (but not somewhere you go for fun). Follow the guidelines very closely. Read the questionnaire before you shop. Pay special attention to anything you need to pick up during the shop (receipts, business cards, brochures, weird photographs). Do the shop. Write the report. Get paid.
Once you do a small handful of shops, you'll know whether this is for you and start exploring other companies. Use this forum as guidance to which companies you should sign up with. You will find that your first month or two is not real profitable as you will spend a lot of time learning the ropes and signing up with more companies. There are about 300 MSC's. I've signed up for a bit over 100. If you do this full time, you'll want to be with over 200 companies - though you might only use 10 or 15 regularly. How much money you make is up to how much time you put into it, how much you are willing to travel and how many other shoppers are in your area.
Hard work builds character and homework is good for your soul.