Hi and welcome to the forum!
If you want the most lucrative jobs you evidently need to purchase your own video equipment (to the specs required by the companies that do video shops) and do those. More realistic for the beginner, however, is to sign up with companies that will have shops anywhere in the country because the shop shippers, gas stations and fast food places. These will allow you to get your start with simple shops that have good instructions as you learn how to do shops. They are also the companies you are also most likely to want to leave behind as you grow more experienced and sophisticated as a shopper. These include Market Force (blue portal), Bare, Corporate Research International and Maritz. Instructions for these types of shops will train you in the basics such as doing timings and descriptions though generally they will not require much narrative. Most companies that require more extensive narratives provide you with a sample that can give you a good idea of the depth required.
What I like about mystery shopping is that I have found a few companies that I am very comfortable working with that my 'style' seems to coincide with theirs. Thus they are happy with me and I am happy with them. I mystery shop for the tax advantages of having my own business and for the reimbursements, which are not taxable as income but are useful to obtain goods and services I would otherwise need to pay for out of my own pocket.