Advice for a new journal editor I was recently appointed to the editorial board of an academic math journal. I'm excited about this position, and I want to do the best job I can. I have no experience with the role however, other than my interactions with journal editors as an author and reviewer. 

What makes a good editor? Those of you who are academic journal editors, what do you wish you knew when you were just starting out?
Here's my advice to you, as a long-time member of editorial boards and, more recently, (co-)Editor-in-Chief: Ask!

Many of us get thrown into this role at one point or other in our lives, and think that we can figure things out, but the reality is that there will be many cases where you simply don't know for sure what to do. In that case, ask around. Ask your older colleagues who've had this role for a while. Ask your editor-in-chief. (In fact, you should have probably asked the questions in your post to your EiC to begin with!) Ask the other members of the editorial board. Don't try to muddle through by yourself, but use the resources you have and assume that the people around you are there to help you figure out what to do.

In addition to this, here are a couple of guidelines you should also follow:


Act ethical: If it is conceivable that a reviewer you have in mind might have a conflict of interest on the paper, don't assign them. If you could possibly be conceived as having a conflict of interest yourself, ask the Editor-in-Chief to assign the paper to someone else. 
More than anything else, the reputation of the journal (and, by extension, your reputation as an editor) rests on whether you turn around papers within the timeline the journal strives for. You will frequently assign papers to reviewers who don't respond to your invitation; or who say that they will do the review, but then don't end up doing in a timely manner or at all. Don't let things drag on. Email them again. If they don't respond, call them. If you can't get a commitment from them, unassign them and assign the paper to another reviewer with the request to turn around the review in a shorter timeline (whatever time you have left after the delay). That will occasionally involve calling in a favor from a friend, and sometimes doing the review yourself. In any case, don't let things sit forever -- problems with timeliness don't tend to work themselves out magically.
If you disagree with a reviewer when you make a decision (say, because the reviewer writes a rather short review and recommends publication; but you feel that the review is not of sufficient quality and want to discard it and instead go with the other reviewer who was more critical), then explain your decision. In fact, explain any of your decisions to both the authors and the Editor-in-Chief. People sometimes think of editorial boards as composed of these inscrutable people who nefariously make decisions in smoke-filled rooms based on their current mood; but in reality, editors just do the best they can with the information they have, and then forget to explain their thinking. In other words, whatever it is that's on your mind, communicate it to the people who might want to know, rather than letting them guess.