I am enjoying reading comics on DCU but since I am jumping around, I feel like there is stuff I don’t know or should know from previous comics.
Should there be an order of comics at least on DCU that we should read or does it not matter in the long run?
You’d spend years potentially catching up to everything that’s ever included in an editors note. The main things to read would be the major universe impacting story arcs. Things like the various Crisis books, Flashpoint, Blackest Night, Rebirth (The Johns runs of Flash and GL, not the new Rebirth stuff), Zero Hour, and maybe even things like Death of Superman and Knightfall are really the big ones (The Crisis books for sure). I felt the same way at one point. Before I wanted to read Rebirth I wanted to catch up with New 52. I almost spent a year “catching up”. By then I was just behind again. I resolved to read major New 52 story arcs (I had already read the stuff I just mentioned). That got me close enough that I was up to date. And for the most part, aside from the major events, little things like [“Superman is off planet right now” *See Superman issue #267, on sale now!] don’t really matter. A lot of times it’s really just fan nods to say “Hey, remember that!” Don’t get me wrong, you should still seek out these major events, as they have a lot to do with character development and such. But from a standpoint of wanting to be “in the loop”, most evens that have happened more than a decade ago are almost footnotes in modern stories.
Personally speaking I’m interested in everything that not only has been created by DC but was owned by the indie comics that DC eventually bought out. For example, I think it was Fawcett that originally created “Capt. Marvel”. The publisher (later DC) that created “Superman” sued because the character “was a copy” of Superman, even though it existed a couple of months before Superman appeared in Action comics, and even had a radio show and outsold BOTH Batman and Superman in the late 30s. DC eventually bought Fawcett but let the copyright on Capt. Marvel lapse. Another Independent bought the title in the 40s. Skip to the late 50s… DC tries to bring back Capt. Marvel but can’t because the license lapsed yet again and Marvel bought it. DC recreates the original story with the new title “Shazam”. I like these historical manoeverings that lead to the environment we have today. I also find it uproariously funny that both “Capt. Marvel” and “Shazam!” have movies that are in theaters at almost exactly the same time 
If you are interested in reading Golden Age comics, a good many are in the public domain and can be found at http://comicbookplus.com/ . I know they have Old Fawcett Captain Marvel anyway.
I’m with @hambo. I started comics about 5 months ago and tried to jump in, figuring I knew enough about the Trinity to get started anywhere. But, most things I read just made me feel like I was missing a lot. So, I went back to the beginning. Even grabbed a few of the non-DC stuff (originally), like Swampy.
@bloodviolet is also correct. Because of dcwiki and several other websites, I was able to read either a summary or comic of every single Jay Garrick and Doctor Fate Golden Age story. It was hard to get to get thew some of the early stuff. Then I decided to read the Watchmen (since it wouldn’t “spoil” anything else). And now I have a new affinity for the cheesy awesomeness of the older stories. I still think Selina Kyle in a dress/skirt is dumb!
Honestly, going that far back really isn’t necessary. Almost everything that happened in the Golden Age has been retconned, and I’d say even a lot of the Silver Age too.
I definitely don’t think it’s for everyone. I’m that way with movies or TV series too though. I HAVE to start at the beginning or I can’t get into them at all. It’s probably my OCD.
That being said, I wouldn’t say most of it has been retconned. There’s LOTS of redundancy. Some of it shows up again and again, with unchanged details. But, yes some of it has been erased from continuity.
I’m very much the same. The summer before The Flash premiered on the CW, I started from the first episode of Arrow (which I had not been watching before that) and caught up. I attempted wading into the vast ocean that is the DC comics multiverse several times (and felt overwhelmed) before I finally stuck around long enough to get somewhat acclimated. Websites, wikis, and podcasts helped a lot. I still have quite a few blind spots and major stories with which I’m passingly familiar, but have never actually read. This service is helping me begin to fill some of those gaps.
Don’t get too hung up on continuity. Read classic runs–Crisis on Infinite Earths, Batman: Year One, The Search for Mera, The Great Darkness Saga, Watchmen, Kingdom Come and Byrne’s Man of Steel–and you’ll be fine. Remember with the constant reboots, continuity becomes somewhat irrelevant. Seek out the good stuff, read it, enjoy it and don’t get caught up in the details.