In regards to Green Lantern, I like diving into various character eras. For GL, I started with Hal Jordan from the Emerald Twilight/Dawn to Geoff Johns’ Rebirth (original) era. While reading Hal’s run I fell in love with his team-ups with Green Arrow. Then I fell in love with Kyle Rayner’s stories from being a Green Lantern to a White Lantern. Now, I’m going deeper into Guy Gardner’s stories.
What helped me was actually googling “recommended reading” for comic book characters. Similar to something like this:
With some comic book characters it’s easier for me to start with the character that held the mantle originally, and then go with the characters that eventually took up the mantle because of the emotional context of the character.