In my opinion, demonstration videos of advanced tactics and knowledge is the most useful thing to show new players by far. For I'm new myself, yet I feel there are so many topics to keep in the back of your mind, topics which also change depending on your current map part location, and topics which also end up being the tipping point between winning and losing, that I believe "Advanced Tactics" in this game are truly Basic Essentials every player ought to know. It never occurs to me GDI can sneak, for example. My new player logic says "GDI = No stealth units = No sneaking without spy crates." But, though some luck was involved, the above video fully demonstrates the opposite. Or what to do with air vehicles, because they die so fast to so many things.
A second, two part example is first, the effects of more veterancy vs. less veterancy, and the effects of crate farming second. Because I remember a game where one speedy Mendoza more or less singlehandedly locked down infantry tunnels for the rest of the match due to having Tier 4 veterancy, from what I partly assume was lucky crate farming, vs. everyone else's Tier 2 veterancy.
Team vehicle compositions and identifying what class/vehicle to bring to a baselock is a third, but by no means final example. I've seen many times where Nod tries to baselock on GDI's front door with Artillery and many repairs, only to suffer a prompt smashing from GDI's more powerful vehicles and the Artillery's weak armor. It wasn't a matter of Nod not having enough money for other vehicles either, because I like to watch team credit counts for donation purposes.
For my own recommendation to new players, begin learning the game by exclusively playing Advanced Engineer for supporting vehicles in field. Offensive repairing is a fantastic way to climb into your team's top 5 players, in addition to easy veterancy points that very commonly build to Tier 3 veterancy that you can then use on a rushing class when the Commander calls for a rush. So far I like this playstyle so much I want to stay with it for the foreseeable future.