Green Lantern is kind of funny in that there was never a reboot after Crisis on Infinite Earths, so you had really old continuity trying to balance in a really clunky way with modern stuff. Rebirth was essentially the reboot mark. It shows snapshots of what the recent continuity used to be for everybody involved, then kicks forward to Everything Changes Now.
After that, you have to decide which character you want to follow. If it's John Stewart, you're kind of out of luck. He's Hal's partner, but his appearances are sporadic and not worth collecting unless you're fanatical--he's the main Lantern in the previous JL cartoon though, so there's a ton of fanfic. Guy Gardner is pretty much spotlighted through most of Green Lantern Corps title, and he's had a long character growth arc through the Giffen-DeMatteis' stuff. Kyle Raynor is Guy's partner in the GL Corps book, but is overshadowed by him IMHO. For some odd reason, I can't stand anything written about Kyle by his creator, Ron Marz, so I don't rec his GL run. Kyle is very awesome in Morrison's JLA, and Geoff Johns writes him pretty good. The Rebirth Secret Files had a nifty Hal and Kyle story that practically screams slash.
If it's Hal and the main stuff you're into, the Sinestro Corps War is probably the best of Johns'. Larfleeze is a brand new character who showed up in the Agent Orange arc of the Light War (I think it's called). I rec the hope arc mostly because watching Hal get messed with is never not entertaining. Johns rewrote Hal's origin into modern canon--traded in 'Secret Origin'.
Pre-Rebirth Hal: Mark Waid writes a fun but hapless Hal in JLA: First Year and The Brave and the Bold (Flash and Green Lantern). The 3-issue mini Tales of the Green Lantern Corps was good (and contains Nekron's intro), . I hold the somewhat controversial opinion that psychotic Hal was great, so I loved Ron Marz' portrayal of Hal every time--Emerald Twilight (Hal goes crazy, becomes Parallax), Emerald Knights (Kyle and time-traveling old Hal adventure and fight Parallax), Green Lantern/Silver Surfer: Unholy Alliance (Kyle is a doofus here, but Parallax and Surfer's interaction are what made both me and Hefner fans of Hal). Afterlife Hal as the Spectre has his best appearances by Peter David, in Supergirl's Many Happy Returns and the issue of Young Justice with Secret the ghost girl. Larry Niven and John Byrne team up in GL: Ganthet's Tale to introduce the rogue Guardian Ganthet and try to stick science in the GL mythos (it didn't work). Neil Gaiman (a Hal fan) got superhero stuff out of his system before he started Sandman when he did GL/Superman: Legend of the Green Flame (posted on S_D a few times). Gerard Jones rewrote Hal's origin in Emerald Dawn, but it's completely out of continuity--it has some fun bits, especially with Sinestro, though, and supposedly the upcoming GL movie will use it.
General GL: 'DCU the stories of Alan Moore' has Mogo's technical origin and the much better portrayal of the Empire of Tears and their prophecy, and Sandman: Endless Nights by Gaiman contains an interesting version of an ancient female Guardian in 'Heart of a Star'.
Just spitballing here
After that, you have to decide which character you want to follow. If it's John Stewart, you're kind of out of luck. He's Hal's partner, but his appearances are sporadic and not worth collecting unless you're fanatical--he's the main Lantern in the previous JL cartoon though, so there's a ton of fanfic. Guy Gardner is pretty much spotlighted through most of Green Lantern Corps title, and he's had a long character growth arc through the Giffen-DeMatteis' stuff. Kyle Raynor is Guy's partner in the GL Corps book, but is overshadowed by him IMHO. For some odd reason, I can't stand anything written about Kyle by his creator, Ron Marz, so I don't rec his GL run. Kyle is very awesome in Morrison's JLA, and Geoff Johns writes him pretty good. The Rebirth Secret Files had a nifty Hal and Kyle story that practically screams slash.
If it's Hal and the main stuff you're into, the Sinestro Corps War is probably the best of Johns'. Larfleeze is a brand new character who showed up in the Agent Orange arc of the Light War (I think it's called). I rec the hope arc mostly because watching Hal get messed with is never not entertaining. Johns rewrote Hal's origin into modern canon--traded in 'Secret Origin'.
Pre-Rebirth Hal: Mark Waid writes a fun but hapless Hal in JLA: First Year and The Brave and the Bold (Flash and Green Lantern). The 3-issue mini Tales of the Green Lantern Corps was good (and contains Nekron's intro), . I hold the somewhat controversial opinion that psychotic Hal was great, so I loved Ron Marz' portrayal of Hal every time--Emerald Twilight (Hal goes crazy, becomes Parallax), Emerald Knights (Kyle and time-traveling old Hal adventure and fight Parallax), Green Lantern/Silver Surfer: Unholy Alliance (Kyle is a doofus here, but Parallax and Surfer's interaction are what made both me and Hefner fans of Hal). Afterlife Hal as the Spectre has his best appearances by Peter David, in Supergirl's Many Happy Returns and the issue of Young Justice with Secret the ghost girl. Larry Niven and John Byrne team up in GL: Ganthet's Tale to introduce the rogue Guardian Ganthet and try to stick science in the GL mythos (it didn't work). Neil Gaiman (a Hal fan) got superhero stuff out of his system before he started Sandman when he did GL/Superman: Legend of the Green Flame (posted on S_D a few times). Gerard Jones rewrote Hal's origin in Emerald Dawn, but it's completely out of continuity--it has some fun bits, especially with Sinestro, though, and supposedly the upcoming GL movie will use it.
General GL: 'DCU the stories of Alan Moore' has Mogo's technical origin and the much better portrayal of the Empire of Tears and their prophecy, and Sandman: Endless Nights by Gaiman contains an interesting version of an ancient female Guardian in 'Heart of a Star'.