Anytime you are beginning a new series, the best place to start is typically the beginning. While Moon Knight appeared first in 1975's "Werewolf by Night," the character didn't get his own solo series until 1980.
The series was taken on by writer Doug Moench and artist Bill Sienkiewicz. This would be the longest run with a consistent writer and art team. Moench's run set the initial framework for other writers to follow when it came to the character having multiple personalities (though it wasn't established as DID, at this point), and this run introduced many of the other characters still seen today, including Marlene, Frenchie, Bushman, and more. While it's unclear which of these characters will appear on the Disney+ series, at this time, it's likely they'll all join the MCU eventually, given their importance to Moon Knight's story.
This is a longer run of comics, and those interested in reading it should check out two Epic Collections: "Bad Moon Rising," and "Shadows of the Moon." While Moon Knight has changed a lot in the decades since these comics, reading the books that first established the basics of the character Oscar Isaac will portray on the screen allows you to develop a good baseline of what to expect — think of it as akin to reading Stan Lee and Steve Ditko's original "Amazing Spider-Man" run.
