Hello and Welcome!
There is a ton of advice to be had here. I strongly suggest that you start by reading the posts in the "New Shopper" area of the forum. The posts there will give you a pretty good overview of the world of mystery shopping.
Other than that, figure out what your goals are. Are you between jobs and looking to make ends meet? Are you employed and looking to supplement your income? Are you retired and looking to have extra spending money and have experiences you would not otherwise be able to afford? Etc. We all have different backgrounds and mystery shop for different reasons.
Once you've got an overview from the New Shopper area and know what you hope to accomplish, check out the lists of companies linked at the bottom of this page - there are over 200 of them. Start signing up and see what they have to offer in your area.
As you get going, feel free to ask questions on this forum. You'll likely get all sorts of answers.
Hard work builds character and homework is good for your soul.