Aside from the wealth of info you'll get from the members here, many of us will be willing to assist in your first few outings to establish / build confidence. It's amazing what a case of beer will get you here.
It's easy to learn but, of course, nearly impossible to master. But that's the fun of it!
7" is what I have on my boat which is sufficient for either sonar or charts, but not idea for both at the same time . While trolling I have my phone in a holder with charts so the sounder is dedicated sonar.