Trump not knowing much about the job should surprise no one who has actually been following this election. He made it very clear numerous times during the debates that he doesn't actually know what the job entails, or its limits.
I'm glad to hear Obama is going to put a bit more effort than is normal to help teach and guide Trump, my hope is that he will continue to talk with Trump after and be a voice in his ear after Trump becomes president, and hopefully be a counter weight to the seemingly very extreme right wing echo chamber that Trump is setting up for himself in the whitehouse. Hopefully Obama can pull him in a slightly more moderate direction.
As far as how much damage Trump can do in 4 years, it's difficult to say. I think the most potential for long term damage would lie with foreign affairs and how the rest of the world might see America if Trump fails to actually earn their respect and earns a leadership role. Then other countries will take up that leadership role on various aspects. We're already seeing this with the Paris agreement for climate change. Other countries are planning on just moving forward without us, including China and possibly even Russia, this would improve the standing of China and Russia in the eyes of the rest of the world while simultaneously diminish our standing.