Starting this with some friends at work tomorrow.
Do any of you use any apps or things to help out or stick to the board counters and processes?
I'd recommend playing 1 game just using the board counters / decks etc so you at least have seen everything in action once, and then use the gloomhaven helper app (non-ios platforms only) to track monster action decks. The helper app also has a neat sync feature where anyone with an android device can connect to it and see all the info and input their initiative values on their own screen.
Gloomhaven Helper
Number 1 thing that stumps everyone... the rule book is always being literal, but that doesn't always mean it's being intuitive/logical. Interpret rules as they are written, don't make any assumptions. (If you're unsure of a rule, search boardgamegeek, the game creator is fairly active there and has responded to most common questions.)
Monster movement rules are the biggest thing people seem to struggle with. They are very literal as written, not always logical in a given situation. For example, if there is no "attack" action on a monster's action card, but there is a "move" action, the monster moves as if it is doing a melee attack, even if it's a monster type that normally only does ranged attacks, which can be super confusing.
This was incredibly helpful to my group when we first started, and we still reference it occasionally.
Monster Movement Flowchart
There's also a scenario app that can help you prevent spoilers when setting up a scenario (just search app stores for gloomhaven scenario app). Game IS balanced around you knowing the room / monster layouts, but my group prefers to not know the actual monster layouts before a door is opened. (Knowing monster types we will run into is fine, just not layouts for us.)
There's an official FAQ on boardgamegeek forums for questions you are most definitely going to have.
There's also a plethora of campaign tracking sheets people have put together on boardgamegeek that we've found helpful. Being able to track what scenarios you've run, and which treasure chests you didn't get and need to go back for etc, has been helpful for later play sessions.
The game can be incredibly frustrating to get into at first if you don't have any experienced player at the table. Stick with it at least until you successfully defeat the first 2 scenarios, and don't be afraid to lower the difficulty level if you're struggling and it's just not "clicking" for your group. (rules for that in the book)