NEW TO COMICS!!!! Having a little trouble getting started

[Dispenser Of Truth]

1. It really depends on the character. Batman and Green Lantern weren't just mostly changed from where they were before the New 52, they actually kept carrying on some of the preexisting plots. Superman and Wonder Woman, meanwhile, got very clean breaks from everything that had come before. And characters like Flash were in a weird bind where the previous continuity didn't apply, but certain aspects of certain stories from it were still very important. Due to mismanagement, it's honestly kind of a crapshoot. Like the others suggested, I'd use Google to find lists real solid starting places (though since you mentioned Hellblazer, a good rule of thumb is that nine times out of ten the Vertigo comic for a character is better than whatever story put them back squarely in the DCU).

2. This is something that varies hugely from reader to reader, and I don't know much of your tastes, but as far as some 'starter packs' for major characters go:

Superman
Superman Birthright, All-Star Superman, Superman: Red Son, Superman: Secret Identity and the currently ongoing Superman: American Alien are all terrific standalone Superman stories that I'd recommend starting with. The version of Superman starting in the New 52 has his origin in Action Comics Vol. 1-3 by Grant Morrison, a very weird but very fun take on his early years. Volumes 5 and 7 of that are also worth checking out, as well as volumes 1 and 4 of Batman/Superman and Superman Unchained (I know that hops around a lot, but it's a little less rigidly serialized than manga due to creative team changes).

Batman
The big 'classics' with him are Batman: Year One and The Dark Knight Returns; I'd also recommend checking out Batman: Ego and maybe Batman: Year 100, as well as Batman: Hush if you want a big crazy popcorn-movie style Batman adventure, and Paul Dini's run on Detective Comics (collected across Batman: Detective, Batman: Death and the City, Batman: Private Casebook and Batman: Heart of Hush) is a wonderful run of short Batman adventures. Any sort of "Greatest Batman Stories Of All Time" collection is a safe bet too, since he's been the consistently best-written major DC character over the years. The New 52 Batman begins with the story Court of Owls, but his new origin is recapped in Zero Year; either of those are good places to start, and all of writer Scott Snyder's work on the character is worth looking at. If and when you've read a lot of Batman stuff and have a good sense of the history of the character, I'd recommend giving the massive run on the character by Grant Morrison a shot, which is collected as Batman: The Black Glove->Batman: R.I.P.->Batman Reborn->Batman vs. Robin->Time and the Batman->Batman: Return of Bruce Wayne->Batman and Robin Must Die!->Batman Incorporated Vol. 1->Batman Incorporated: Demon Star->Batman Incorporated: Gotham's Most Wanted. If you enjoy that run, check out the book Grayson, which is an excellent de facto sequel.

Wonder Woman
She's harder to get hooked on, as she hasn't been treated very well over the years; your best bets are either Greg Rucka or Gail Simone's takes on the character. The current ongoing Legend of Wonder Woman origin for her is great, though, and may be the best story with the character yet; this is where I'd start. The New 52 version of her had an extended run by Brian Azzarello and Cliff Chiang (Wonder Woman Vol. 1-6) that's very controversial - many, including myself, really didn't like it - but is also very popular among new readers, so I'd give that a shot too.

Green Lantern
The big choice with him is Geoff Johns Green Lantern epic beginning with Green Lantern: Rebirth, running for years on end with big stories including Sinestro Corps War and Blackest Night (it continued for years after Blackest Night, but I personally think it was all downhill from there, a not uncommon sentiment, so beware). However, I think the best starting point for the character is actually the book DC: The New Frontier, a DC 60s period piece on the formation of the Justice League with him as one of the two main characters. I'd also check out the recent Green Lantern: Darkseid War one-shot (while an event tie-in, I assure you it works perfectly as a standalone story), and I've heard good things about Green Lantern: Willworld. Emerald Dawn is also commonly recommended for new readers.

Flash
The best stuff with the character actually isn't with the version of him currently running around, but actually the previous Flash, Wally West. I'd check out Flash: Born to Run and Flash: Return of Barry Allen, and maybe the recent Flash by Grant Morrison and Mark Millar collection. If you're really interested in Barry, the place to start is either Flash: Rebirth, or the first volume of the New 52 Flash.

Justice League
Along with New Frontier which I mentioned before, the biggest and best run on the team is by Grant Morrison, collected across JLA Vol. 1-4 (the followup run by Mark Waid, collected through the back half of volume 4 and volume 5, is also a lot of fun). JLA: Year One is an outdated version of their origin not focusing on the 'big three' of Superman, Batman and Wonder Woman, but is a great character piece (and followed up by a sequel titled Brave and the Bold, which focused on Flash and Green Lantern). Identity Crisis is...not my cup of tea, but a lot of new readers love it, so I might check it out too, though be warned it gets astonishingly brutal. The New 52 run by Geoff Johns is fine popcorn fun, and can be read in order from Justice League Volume 1 onwards, though I think the current JLA title by Bryan Hitch is a lot more entertaining.

Additional Recommendations (note: a bunch of these are from Vertigo or Wildstorm, and therefore not part of the larger DC Universe; I'll bold the ones that are part of DC proper for you)
Watchmen
Swamp Thing by Alan Moore
Kingdom Come
The World's Greatest Super-Heroes by Paul Dini and Alex Ross
Preacher
Ex Machina
Planetary
Tom Strong
Top 10
Sandman
Hitman
Hellblazer
Lucifer by Mike Carey (this and Hellblazer have one toe in DC because of their connection to Sandman, but events in those books are not reflected by other DC books)
Astro City
Starman
WE3
Gotham Central

Of course, DC is also about to put out a number of new #1 issues with "Rebirth", so that will likely offer a number of good jumping-on points too, even if some are probably going to be much better and more new-reader friendly than others.

3. There are two "Infinite Crisis" stories. The recent one was a video game tie-in by a pretty good writer, so I assume it's both accessible and fun. The big one by Geoff Johns is pretty fun, but way too bogged down in existing history for a new reader.

As for the other ones you asked about, I wouldn't give Injustice, Flashpoint, or Knightfall recommendations - the former is controversial at best, the middle one is mediocre, and the last is okay but distinctly of its time. Also, I wouldn't try to just read Batman from #1 to #700 - time and money involved with that aside, it starts in the 40s and it takes decades before those books start to look in any way modern. If you're looking for a Batman book to read through from beginning to end, other than the current New 52 Batman, I might recommend the title Batman: Legends of the Dark Knight, which probably has the best hit-to-miss ratio of any of his major ongoing titles over the years.