I mostly play the soldier because the skills really transfer well and its my strongest class. I also play the demoman and the scout, but I dont play off-classes unless the team gets into a situation where it would make sense. (which is barley ever) I know how to play demoman and scout and medic but I think I am not very good at it. I have less than 100h in the game, maybe like 70h from playing pubs once a month when I was bored.
I'm still trying to get out of the lower ranks, since right now I keep getting into games where people will play off-classes and not use their mics.
there are probably more videos out there that might be a lot better, you can also watch povs of matches and stuff and learn from them. It's been 5 years since I've touched TF2, but 6on6 is great fun.
Every time I think of installing TF2 I remind myself of the 15GB download. That's just outrageous. This cannot be a well running game. So much crap in it.