Here are some things I didn’t know right away and/or tips that help me now.
Do the main campaign and explore the map, find all the tenets and altars early on. You do it once and all the major benefits (12 skill points 28 paragon points) transfer over to your other characters forever.
Try seeing how far you can go with your own build. This means reading the aspects, affixes and runes, rather than copy and pasting from the internet. It really does help you learn the game.
Learn what the vendors do. A lot of people I run into don’t know something like how to reset masterworking, change affixes, combining useless runes, making gem fragments, that mythics can be made or be random.
Quit doing boss lairs and hordes early on. (Unless they are for grim favors) Now I’ll get some hate on this one but imo you should avoid doing more than a handful of boss lair fights until you’re ready for t4, depending on the character, probably level 180-220. Hordes and boss loot are nerfed a lot if you are doing them in t1-3, plus you gain no XP in boss lairs. There are much better ways to spend your time getting gear and gaining experience. You don’t need mythics super early.
Vary how you play. There are different options of what you can do, undercity is my favorite, the seasonal quest is good, helltide is great, regular and now escalating dungeons have good benefits especially for lower leveled players.
Also focus on doing grim favor events, you get a nice XP boost for completing any of them and then an even nicer XP boost for completing 10 favors, on top a a cache. Plus there are a lot that can be done very quickly.
If there is something you can’t do with your character try to find a variation that is similar and more effective for the task at hand (I.e. have a speed farmer and a boss killer). You can save multiple builds in the armory so you don’t have to reset skills and paragons over and over.
Read. The game explains relatively clear what most things do, you just have to actually read it.
Level your glyphs up to 15+ very early on and 46 before you try to do a whole lot in t4. Those are big boosts.
You don’t need anything past 1ga gear to get to t4. Probably don’t even need that in all honesty but it’s hard not to get a 1 ga that is useable.
Use your orbs, I personally forget about my orbs all the time, especially once I’m t4, but they might as well be spent.
Use elixir and 3 incense all the time. You can use one incense from each category, meaning you can give yourself a pretty big boost just by utilizing 4 things at once. XP doesn’t stack on these but everything else does.
Don’t masterwork 12/12 until you have a piece of good gear. First it’s not completely needed early on and secondly obucite isn’t the easiest to get early, but it only costs like 1100 to go 8/12 compared to about 6000 for 12/12. You do get it back when you destroy a piece of gear with it tho, so it’s not the biggest deal as long as you aren’t resetting masterwork over and over to hit the right affix.
If you only play solo that’s great, that’s mostly what I do, but if I you are more social or want to level quicker, the party finder is great, trade channel can be interesting and I’m not a discord guy but I’m sure there’s lots of information in there. Also there are a lot of players like me who will help a struggling player get some gear or help them in a harder difficulty. The D4 community is actually pretty helpful, regardless of what you read on here, if you are actively seeking help.
You don’t have to do every season, I play more than most people can, so I understand it can be very difficult to get a character really good if you can only play a handful of hours a week. But if you want to keep grinding away at a character until 300 at your own pace this is an option in eternal. Could eternal be a little better? Sure, but in all fairness the vast majority of players are starting with a new guy every season.
I’m sure there are more helpful hints that other people have as well. Feel free to share or call me an idiot for enjoying this game, either way hope you all have a wonderful day and good luck looting!