I don’t have a lot of knowledge about green lantern other than just general stuff about the his origins. What do I need to know before starting the Geoff Johns run?
Nothing. Just start with Rebirth and you’ll be set.
Green Lantern Rebirth by Johns and Van and Van Sciver is an excellent series and and a great jumping on point.
I’m a guy who has some criticisms of that run, but even from my position, I would agree with all of the other posters here.
Just jump into that Rebirth story and then into the Geoff Johns run on the Green Lantern title. Sure, there are references to older storylines, but if you go to those storylines, they will refer to other, older storylines. If you feel like you have to know everything, you’ll be going down an unnecessarily deep rabbit hole.
Start with Rebirth. It will refer to older stories, but just accept the final, simple references of the expository flashbacks and move on from there.
EDIT: Those simple, expository flashbacks that I referred to: I forgot to mention that they are well-written. They are written intentionally for readers just like @Greenbooks82, for people that don’t walk the world with deep knowledge of the GL comic books. It is a very welcoming and friendly open to Hal Jordan’s return. Those criticisms I said I had have NOTHING to do with Rebirth. It is excellent.