I can't help you much on Flash, but I do know that the Mark Waid run in the 90s was pretty cool.
As for Green Lantern, it matters on what era you really want to get into:
If it's the Golden Age, then there's DC Archives, unless you've got a lot of dough to spend because Golden Age Green Lantern are some of the priciest issues out there. All-American Comics #16 is the first appearance of the original Green Lantern (Alan Scott), and then he gets his own self-titled comic (Green Lantern (vol. 1)) starting with #1, naturally.
If it's the Silver Age, then I'd suggest starting from the beginning: Showcase #22 (there are plenty of reprints if price is a problem) then of course Green Lantern #1 (vol. 2) and continuing from there. There are DC Archives of these, as well.
For Post-Crisis Green Lantern, then you need to start with Green Lantern (vol. 2) #193-#200 and Green Lantern Corps #201. These are essential for the changes that happen from the Crisis, and then the post-crisis set-up for the GL Corps. Then you need to immeadiatly read Green Lantern: Emerald Dawn (6-issue mini-series) which is Hal Jordan's Post-Crisis origin. Emerald Dawn II (also a 6-issue mini-series) is a nice add-on to it if you enjoy Emerald Dawn I, but not necessary. It's the Post-Crisis origin of Sinestro and Hal's first meeting with him.
If you're into Kyle Rayner, then Green Lantern (vol. 3) #46, #48-51, and #54 are absolutely essential issues. It shows the downfall of Hal Jordan as he's overtaken by Parallax then destroys the Green Lantern Corps and Guardians of the Universe. Followed by the origin of Kyle Rayner, and in issue #54 is the reason he REALLY
decides to get responsible and become a hero.
And finally, if you want to just follow the modern Geoff Johns take on Green Lantern, then I'd start with Green Lantern: Rebirth #1-6 which re-introduces Hal Jordan and Guy Gardner as active Green Lanterns and returns Sinestro to the DC Universe as a major villain. After, just follow the superb Green Lantern (vol. 4) series which continues straight into the currently running Green Lantern (vol. 5) which was changed VERY
little story and continuity-wise by the New 52 changes. The Secret Origin storyarc in Green Lantern (vol. 4) is supposedly the modern origin of Hal Jordan, but in reality it's more like the set-up to the Red Lanterns' introduction into the Green Lantern mythos.