Zarstuff
A lot of this is bad advice and/or down to personal preference. There are high-level players that use default hud, viewmodels on and/or change their sensitivity every other day et.c. Find a setup that's comfortable for you.
I'd really advise against blindly picking a main class without fully understanding the meta and the roles of the different classes in 6v6. Play the newbie mixes on a few different classes and get a feel for 6v6 before setting your heart on one (however, in my experience players from a cs background tend to favour scout). Even then, plenty of people change main class after years of playing one.
Also, you don't need a mentor. It may be useful to have someone to answer any questions you have. BUT I don't think someone spoon feeding you 6v6 will actually help your understanding. The simplest way to learn is simply by playing, and every time you die. Question it. Think about why you died and the mistakes that led to it, as long as you don't assign it to some bullshit out of your control (omg I lagged; he's cheating; what an idiot; blahblah) then you will improve.