That is a can of worms about on par with a leftist asking "In what order should I read theory"? There a 1001 correct answers, and there is no exact timeline about how you can delve into the character.

I have a decent bit of superhero knowledge and I dive into it every now and then, but I will not claim that I am some absolute authority on this subject and that I can give you a definitive list.

I will however recommend "Batman: Year One" as it is a good place to start with the character and that is considered one of the best comics of all time.

For a Batman/Joker story I would recommend the "Killing Joke" series, but I will say that is is fairly mature in its content, as it really attempts to delve into the Joker's psychology as a tragic but evil character.

The Batman stories in the "Flashpoint" Series is fun as a side story as it follows a world where the Flash makes one change and alters all of history, and Batman ends up being Thomas Wayne (the dad) rather then Bruce Wayne (the son), and this is a much more aggressive and violent Batman, with a pretty shady moral code as Thomas goes full capitalist and is pretty evil.

The Long Halloween is also pretty good!