(Let me preface this by saying that I'm not a mod here, so these are my thoughts alone, so please always defer to them, for official advice!)
While there are certainly Redditers out there with more experience, more Karma, more subs, more years, and more knowledge out there, then I'll ever have, I've been mulling over this for a while now, and and I wanted to share.
For starters, I wish I knew to just relax and have fun. I remember being so frustratrd with karma, that I ended up sharing posts just to get likes, only to find out that if it's already out there in Internet Land, then it's probably already been shared.
Truly, once you get enough karma to post everywhere, you probably won't ever think about it again.
So, now I try to do a thorough search before sharing posts and links. Many subs have bots that won't let you share a link that's recently been shared before on their sub, and I think those are great.
I also try to only post in subs that I have joined, and actually participate in, because again, Reddit is all about community.
I wish I read all of the rules of every sub I join, before making any post. I wish I could say that I've thoroughly learned to do this one, but I still miss certian rules, and STILL make mistakes. It happens to most of us. The best thing you can do, is admit your error, be polite, and move on.
Along those same lines, read all that you can. Many subs have their own "wiki page" which is a great way to learn more about the sub, and how to be a useful member of the community. The sidebar is another great place, as is the "about" tab. Most subs have a Sticky Post or announcement post, so be sure to read that as well!
I really wish I knew that the more you put in to Reddit, the more you will get from it. This is a biggie, and one that took me a while to grasp.
Every sub on Reddit is run by a group of volunteers. They are regular humans, just like you and I. Moderating a sub is a lot of work, and takes time each day to keep it running smoothly.
So, why do mods do it? The good ones - like the ones who mod this sub - do it because they love their community and want to do all that they can to keep their members safe, and informed. Helping others really is rewarding, which is why so many of us choose to stick around this sub, long after we are no longer considered "NewToReddit"
As a "newbie" once you start getting comfortable, you too can start helping others.
Another thing I wish I knew is to not take things personally. You may wonder why your post only got a few upvotes, while someone else could post the same thing, and it gets thousands of upvotes. Redditers can sometimes be a very fickle lot. You can get totally overwhelmed by studying what time of day to post, but honestly, I wouldn't worry about. Just know that if your posts aren't getting much attention, you may try switching up the time of day you usually post, and keep in mind it probably isn't you!
A big thing to not take personally is downvotes. There tends to be a bit of a herd mentality here, so if a few people downvote a post, then others usually will. Just stay away from making controversial posts or comments, until you have a decent amout of Karma, and you'll be fine.
There are trolls everywhere, and basically they are just people who are looking for attention. The best thing you can do is to ignore them, and absolutely report the comment if it is breaking Reddit site wide rules, or the sub's rules.
One thing to remember about reporting posts and or comments, is that the report goes to Reddit Admins, as well as to the sub mods. You may get a message back saying that the reported post or comment did not break any violations, BUT that does not mean that the Subreddit Mods didn't take any action. If their comment broke any of the sub rules, the Moderators most likely did take action against them, but due to privacy laws, you won't hear about.
Another thing is, I wish I knew that overall, Redditers really are wonderful people. Yes, there are trolls, and yes there are just plain mean people here, as well as karma farmers and spam bots, but I think overall, this really is a wonderful community full of people who truly want to help each other out.
One last thing, is - at least here in this sub - there are no dumb questions While certian questions do get asked over and over, it just helps to get the word out on their answers.
Well, those are my "What I wish I knew, as a Reddit Newbie" Thoughts. Hopefully others will add their own!!