I'm by no means a veteran but I've played a few leagues now and the main points I would say are:-
1) Unless you are very patient and don't mind making characters that are not viable at end game, do some research and find a guide for a build that will work at high levels. I've used these 2 in the past leagues
This is the one I'm playing at the moment
Spectral Throw - https://www.pathofexile.com/forum/view-thread/1839087
I'm currently up to T13/14 maps and it's still going strong.
This one is a bit tankier and very easy to gear, but I found it struggled at the very high levels with dps against bosses, its really and AoE clearing build to get you started.
Stun Sunder Beserker - https://www.youtube.com/watch?v=GRKLks6sq8Q
2) Don't ignore life and resistances. You cannot glass cannon your way through this game, it will get very frustrating.
3) Use a loot filter. Filterblade is the best site to use - http://www.filterblade.xyz/
4) Use trade sites to help understand the value of your loot, as well as find items to trade with other players. I use http://poe.trade/ for items and http://poe.ninja/challenge/currency for exchange rates. You can also install the trade macro to help price things quickly. https://github.com/POE-TradeMacro/POE-TradeMacro/releases/tag/v2.8.0
5) Don't ignore the usefulness of flasks, they are very powerful and many different effects can be mitigated or removed using them. The suffixes 'of staunching' (remove bleed effects) and 'of heat' (removes freeze effects) become critical.
6) As stated above, don't bother trying to craft anything complicated, it will cost you more than just buying what you want and unless you know what you are doing, you are very likely to screw it up.
7) Remember that skill gems level up as you kill stuff. As they gain experience and level up, the effects become more powerful. You can also level gems in a secondary weapon set (press X to switch). This becomes important when going for 20% quality gems as you will likely need to level up every gem you own twice.
8) Don't play hardcore until you know what you are doing.
9) Play the current league, there is no penalty if you are starting from scratch and you may as well enjoy the league specific mechanic.
10) Do some research into the masters, as they will give you several benefits not least of which is your own 'hideout'. You need to complete their missions to level them up so when you encounter them in the wild, make sure to do their missions.
I could go on for pages and pages but to be honest, the best way to learn is to play and learn from your (sometimes costly) mistakes.
1) Unless you are very patient and don't mind making characters that are not viable at end game, do some research and find a guide for a build that will work at high levels. I've used these 2 in the past leagues
This is the one I'm playing at the moment
Spectral Throw - https://www.pathofexile.com/forum/view-thread/1839087
I'm currently up to T13/14 maps and it's still going strong.
This one is a bit tankier and very easy to gear, but I found it struggled at the very high levels with dps against bosses, its really and AoE clearing build to get you started.
Stun Sunder Beserker - https://www.youtube.com/watch?v=GRKLks6sq8Q
2) Don't ignore life and resistances. You cannot glass cannon your way through this game, it will get very frustrating.
3) Use a loot filter. Filterblade is the best site to use - http://www.filterblade.xyz/
4) Use trade sites to help understand the value of your loot, as well as find items to trade with other players. I use http://poe.trade/ for items and http://poe.ninja/challenge/currency for exchange rates. You can also install the trade macro to help price things quickly. https://github.com/POE-TradeMacro/POE-TradeMacro/releases/tag/v2.8.0
5) Don't ignore the usefulness of flasks, they are very powerful and many different effects can be mitigated or removed using them. The suffixes 'of staunching' (remove bleed effects) and 'of heat' (removes freeze effects) become critical.
6) As stated above, don't bother trying to craft anything complicated, it will cost you more than just buying what you want and unless you know what you are doing, you are very likely to screw it up.
7) Remember that skill gems level up as you kill stuff. As they gain experience and level up, the effects become more powerful. You can also level gems in a secondary weapon set (press X to switch). This becomes important when going for 20% quality gems as you will likely need to level up every gem you own twice.
8) Don't play hardcore until you know what you are doing.
9) Play the current league, there is no penalty if you are starting from scratch and you may as well enjoy the league specific mechanic.
10) Do some research into the masters, as they will give you several benefits not least of which is your own 'hideout'. You need to complete their missions to level them up so when you encounter them in the wild, make sure to do their missions.
I could go on for pages and pages but to be honest, the best way to learn is to play and learn from your (sometimes costly) mistakes.