Welcome! I've learned a lot from this forum and I'm still learning every time I log in. It's great that it's free and it's set up so it is self-service and you can read and learn at your own pace. Since you found the New Shopper Area, you have probably read all the threads above the one you started, so you probably already have a good grasp of what mystery shopping is and how it works.
The more you read, the more you will learn. Sign up with companies and try some shops. You learn by doing. Be sure to read the guidelines and do exactly what the guidelines specify.; If you make any changes to the requirements, your report likely will not be accepted. A good company to start with is Market Force, because they are newbie-friendly and their assignments are almost all pretty easy and their reports are short and simple with almost no narrative required.