To first answer your question directly: I was employed by a company using the product and got started by observing existing configurations and the changes and operations more knowledgeable colleagues made.
However, McAfee does offer an NGFW training course which should give a good starting point, it takes four days. Please check out Educational Services. This is the US page but the course is available elsewhere.
Please join another NGFW group.
From that group, you will find some useful basic information to help you understand NGFW quilckly.