Short answer: Don't worry about it, most stuff isn't shared and the stuff that is no one will mind if you take it.

Long answer: Most things are either re-useable or character specific.
Books you read will still be on the shelf for anyone else who comes along as (unfortunately) you can't take them, only read them and put them back. (And two or more people can read from the same shelf at the same time.)
Enemies you kill award experience and loot to each person who did enough damage individually. Everything you can see on the body is yours, even if you left it there no one else could take it.
Containers like baskets, wardrobes, crates etc. are also individual - you can take everything out and there will still be stuff in there for other people to take.
Quest objects - for example if you have to retrieve a sword from a tomb you can take it and it'll still be there for anyone else who needs it, even if they come in while you still have it in your inventory.

The exceptions are crafting material nodes, like iron ore, jute plants, nirnroot, butterflies and so on, runestones and treasure chests which have to be unlocked. You can tell these work differently because they disappear after you've looted them. But they're treated on a 'first come, first served' basis - if you find it then it's yours to take. There are plenty of them around and they respawn after a while so no one will miss out completely.

There is sometimes debate about nodes which contain 2 or more items though. If you only take 1 thing the node won't disappear and therefore won't 'restock'. Some people say you should take everything, even if you don't want it, because chances are other people want the same bit you do. Other people say you should only take what you need because there's no knowing what other people will want. But I don't think there's a clear right or wrong answer here, so it's up to you what you do.

Books, the bags barrels and crates you loot Provisioning stuff from, and things like the weapon racks in towns - these are instanced to you. Only you can take those items and it doesn't affect anyone else.

Rune nodes, alchemy nodes, cloth nodes, ore nodes, wood nodes, treasure chests - these are the same for everybody. If someone else gets to that node before you, they get it and you don't. (EXCEPTION - chests, Heavy Sacks in Dungeons and Trials - these everyone in the group can loot. So call them out when you see them! )

Monsters everyone sees, but the loot is instanced to you.