Hard to say if you're really starting from scratch. I think
@Eboue has once put together a quite handy list of games you'd need to watch if you wanna get into the NBA stuff, but can't find it right now.
I'd suggest to read up a little bit about the history of the league. If you wanna dive into that, can't go wrong with the historical rivalries and franchises (Lakers, Celtics, Bulls a bit more recently). You could read up about the great players of the past, and find some highlights of them on YouTube (just go by some "Greatest" lists, MVP list, stuff like that) to see what kind of style of players you like most. Maybe try to pick a team that you like (jerseys, city location, whatever) and read up about their history and current players. If you wanna start more recently, the Warriors of the past half decade are worth watching for a bit, and most LeBron James teams too.
Hard to say which classic games are must-watch because there are so many, and it depends where your focus is. I'd definitely start with Finals or at least playoff games because the intensity is much higher than the regular season and games mean a lot more as well. Game 7s are always fun and intriguing (e.g. 2019 Philadelphia-Toronto Game 7, 2016 Finals Cavs-Warriors Game 7, 2010 Finals Celtics-Lakers Game 7, 2013 Finals Game 6 and 7, ...). All-Star games might be a good start to learn about the stars of the league but (apart from 2020) they've been boring as hell for the last decade or so.