edit: I added a few more things.
Here are a few general tips for new players based on public information and also how things work in PoE1. It's nothing that I would consider spoilers, since it's very basic information. This will be helpful to save some headache early on especially if you're used to the way Diablo 4 works.
Each level up grants you one skill point on your passive tree. These can be allocated to attribute nodes (also known as travel nodes) or other nodes in clusters that boost particular other stats. You might be tempted to go straight for the +10% damage nodes from the start, but generally speaking, most players will eventually end up respeccing it to take more attribute (travel) nodes so that they can path to more substantial passive nodes that are further up the tree. This is the decision making gameplay loop - Do you take more attribute nodes to get to high level passive nodes earlier, or do you take the low level passive nodes now and wait until you have enough money to respec later?
Strength grants you life, Dexterity grants you accuracy, and Intelligence grants you mana. There are also attribute requirements on gear. High armor gear requires high STR. High evade gear requires high DEX. High energy shield gear requires high INT.
You gain different skills by consuming something called an uncut skill gem. Then you can choose any skill from any class to use. Some are restricted by weapon type. Generally, you want to stick to your own class' skill set in the beginning, but feel free to experiment if you're feeling adventurous. Each skill can be modified two times with a support skill gem that you can obtain by consuming uncut support skill gems. You can only use one support gem on one skill at a time, so it's not like you can put the same strong support gem into every skill that you use. You will have to think hard about which characteristics about each support gem fit the skill it's matched to as well as your playstyle. Support skills work sort of like how legendary powers work in Diablo 4.
Some skills are better suited for single target DPS and some skills are better suited for AoE DPS. This can also be modified with the right support skills. Keep in mind what is the right skill for the right job if you're fighting a boss or trash mobs.
Some skills have cool interactions with each other. For example if you fire projectiles through a flame wall, your projectiles will get bonus fire damage. This is cool in party play. Experiment to find more cool combos
The NPCs in this game are actually worth a damn and carry good gear. Check back periodically to see if they have anything new because they can randomly have godlike rare items in the shop. For weapons, keep an eye out for +damage or +skills. Like in most ARPGs, keeping your weapon updated is very important in early game progression.
Crafting is a little complicated, but don't worry about that in the beginning. Just know that you can use consumables to turn normal items into magic, and magic into rares, so if you see a good quality normal item or magic item drop or in the shop, it's still valuable to you.
Wudijo made a crafting guide.
edit: more stuff
If you die, you respawn at a checkpoint and all other mobs respawn too. This is the penalty for death. It's actually not that bad because if you died you might be underleveled, and clearing the instance again will get you more EXP. If you die on a boss, you will have to restart the boss fight. You can't portal out during a boss encounter. If you want to manually reset an instance for farming purposes, ctrl+click on a waypoint.
Due to server limitations, if you are idle in town for too long and the dungeon you are in has no active players inside of it for 8 to 15 minutes, the server will close that instance.
If you want to link an item in chat, ctrl+alt+click.
There is a weapon swap feature in the game where you can swap between two different sets of weapons. If this is too complicated, don't bother with it. If you want to min/max, this will be useful. As you progress in the game, you will acquire the ability to split up your skill tree with weapon set passive points that only apply for when you are using a particular set of weapons. For example, if you want to be a mage with both fire and ice, you can have a staff with fire buffs and a staff with ice buffs and swap between them, and assign your fire skills and fire passives to the fire staff and the ice skills and the ice passives to the ice staff. Or, if you want two different weapon sets for single target DPS and AoE DPS, that is possible too.