Hello and Welcome!
First, mystery shopping is a great way to add to your income without interfering with your full time job and having a very flexible schedule. That's exactly what I do and I love it.
Start by reading the "New Members" area on the forum. The world of mystery shopping is a lot larger than most folks think and has plenty of pitfalls. Sign up for the most commonly mentioned companies on the forum to start - they are all larger companies and will have shops pretty much everywhere. Thereafter, you want to join a few companies each week until you have more work than you can handle. Feel free to ask questions here along the way. You are almost certain to get multiple answers as we all use mystery shopping a little differently.
Hard work builds character and homework is good for your soul.