The common mistake that most beginners in Path of Exile make is assuming that it's like most other MMORPGs, and it is in a lot of ways. But in terms of the core system, it's quite different from the others in a lot of ways as well. The skill system is divided and laid out in a different way, the way wealth works here is not the same, and the Path of Exile items have their own ways that differ them from those in other games. Therefore, playing this game is about relearning the fundamentals of MMORPGs, and here are a few things you should remember.
Follow a passive skill build
Perhaps this is just like that of other games, but not quite exactly the same. Since Path of Exile has a separate passive and active skill system, it's not exactly how you'd approach it in other games. In Path of Exile, being well-rounded holds more weight since survivability is more important here than that in other games. A dying character has a panicking player behind it, and a dead character doesn't deal damage at all. Picking the right class for your intended style of play is crucial.
Grind experience in groups
When grinding with groups, you breeze through areas and kill monsters more quickly, which lets you get more items and experience in a shorter amount of time. Remember that Path of Exile is supposed to be a cooperative game, so soloing all the time is actually playing the game wrong. Unfortunately, there are still flaws in the game's loot system, so there's quite a bit of loot stealing. You'll have to get used to it until Grinding Gear Games comes up with a more decent system. Besides, you may be able to pick up someone else's loot anyway, so you just have to be vigilant.
Pick up all orbs
Take note that any Path of Exile item that contains the word "orb" in it should get your alarm bells ringing in your head. Whenever you see one dropped, pick it up immediately and keep it for it will be your major item for trade. Orbs are what is known as currency items, which are the items mostly used for trade in Path of Exile, especially since there's no gold here. Even common ones like Orbs of Alteration and Chromatic Orbs should be picked up. Once you start getting old of 6-linked items, you may spend tons of these common orbs in order to get the right socket color for your items.
Pick up items with linked 3-color sockets
If ever you encounter an item dropped with 3 linked gem sockets with different colors, take it and sell it to a vendor. This is an easy way to get a Chromatic Orb, which will be your staple for trading and getting better items in the game. Upon reaching level 50 at least and you're thinking of grinding Merciless, review on this and read up on all the items you should be picking up. Keep an eye for Path of Exile items that has 5 or 6 sockets, even if they're just the white ones since they very valuable.