Trying to come back but feel so overwhelmed

When trying to create a character I feel so overwhelmed as to what I should or should not build to be end-game viable. Could anyone please suggest the fundamentals of how I can build but not be wrecked end-game? Thank you

Well, just getting a good amount of life or ES, capping your elemental resists, getting decent gear and properly supporting your main skill will have you not getting wrecked. If you also mix in some more defensive mechanics and supporting skill setups you'll be comfortable for most of the endgame. Advanced skill, passive and gear synergies are required only if you want to do something unusual or clear all content in the game, you can normally keep it pretty simple by going only for obvious passives and rare items.

The inevitable follow-up question to these threads is "what are good benchmarks for 'enough life/ES, decent gear, proper support, etc.'???? What's enough DPS, what's enough armor, what's enough evasion? HOW DO I TELL WHEN MY BUILD IS GOOD?!?!"

I'ma pre-empt it this time - there are no benchmarks. Not really. Builds vary so wildly in this game that there's not really any universal plan to Bild Gud. I'll try and condense some of the closest-to-universals for you, though.

1.) Life-based builds are cheaper, easier, and generally more forgiving for players without a lot of game knowledge, or who're rusty. A life-based build should try and get to 5k life as a generally accepted minimum by "Endgame" (roughly level ~80). More is always better, but don't overextend to grab every possible life node in the tree. A decent rule of thumb is that if you're spending more points traveling to the life cluster than you are grabbing nodes in that life cluster, then that cluster is too far away to be worth doing.

If you pound out a billion experimental trees on poeplanner the way I do, you tend to get a feel for more efficient pathing that gets you near the stuff you need naturally, but in the interim: shoot for 150% life in your tree minimum, try for 5k life between tree and gear, and don't spend more than three points getting to a given life cluster.

This is as close as you'll ever get to a universal benchmark in PoE. Everything else is much muddier.

2.) 'Enough DPS' is the point where you can clear whatever content you're targeting without straining your sustain (i.e. your ability to keep yourself alive in the face of incoming damage and to keep your skills supplied with sufficient mana). If your primary means of sustain is flasks (as they usually are for a lot of newer or rusty players), then you need enough DPS to kill what needs to be dead before your flasks run dry. If your primary means of sustain is leech (common for a lot of attack builds), or regeneration (less common and highly undervalued), then you need enough DPS to drop whatever needs dropping before they can outpace your sustain and eat through your buffers of life and mana.

Generally:
*You should never feel threatened by a typical white pack, or a typical blue pack. These are delicious Scooby snacks to be greedily devoured, not 'threats'. if your damage is low enough that regular packs are screwing with you, this is a HUGE red flag. Most Bloodline packs will be the same way. The only things that should threaten you will be on-death effects like Corrupted Blood or [X] Bearers.

*You shouldn't be threatened by most individual rares without a Nemesis mod. The only exception are reflect rares, which won't threaten you unless your DPS is too good. If singleton normie yellow rares are giving you a hassle, again - red flag.

*Bosses should visibly be losing health while you're attacking them. Most bosses a returning player acclimating to the game again should be concerned with shouldn't take more'n a minute tops of sustained punching to go down. Izaro, mid-tier map bosses, act bosses, all those schmucks should be pretty clean and easy, and obvious to tell that your damage is having an effect.

If any of these are untrue? Not enough deepz.

3.) Your main skill is usually in your chestpiece, and is expected to be at least five-linked (i.e. five linked sockets). 6-links are very nice when you can get them but tend to either be cruddy (random Izaro corrupted 6L drops) or hellaciously expensive (uniques or nice rares someone actually managed to fuse). For a returning player, plan around a five-link. Typically you want to focus your main-skill links on MOAR DAMAGE. Support and utility stuff is nice but as my brother's fond of saying, the strongest status effect you can inflict on an enemy is 'Dead'.

Try for at least two 'More' modifiers in your gem link set-up. The specific language matters; 'More' is vastly more powerful than 'Increased'. All of your 'Increased' modifiers sum up into a single damage multiplier, but every single 'More' modifier is its own separate, compounding multiplier that applies after your total 'Increased' damage. Once you have your main skill and the two best 'More' gems for it (or more, if you're using a skill that can benefit from more than two!), you can look into more utilitarian supports. Most of them are...kind of crap, but certain ones can enable builds or accentuate what you're doing, one typical example being Greater Multiple Projectiles or things like Physical to Lightning that modify the base properties of a skill.

If you're playing a two-skill build (i.e. a build that uses one skill for general pack/mob clearing and a second skill with better single-target damage for killing big bosses, with a very good example being Kinetic Blast/Barrage wand characters), your boss skill goes in your 5L and your clearing skill goes in a 4L. Otherwise, the same rules apply - try to fit at least two 'More' modifiers per damage skill, and keep non-damaging supports to a minimum. Generally only one, if you can manage it.

4.) Spell builds are easier than weapon builds. You don't have to worry about accuracy, and the primary way you scale your damage is by gem level so you don't need to continuously upgrade your weapon, or find the money for new ones on poe.trade. Contrary to the game lore, all seven character types are equally capable of utilizing both spell and attack builds, including the Duelist and Marauder.

5.) Ascendancies. If you're not familiar with Ascendancies from when you last played, I very highly recommend reading up on them, because they're critical to builds these days. Selecting the proper Ascendancy for your intended skill or interaction is crucial, and it also cannot be respecced if you decide to start as a Templar, say, but then decide later that a Witch Ascendancy would've been better for your build. That's a reroll. A returning player should try and bank on getting six of the eight available Ascendancy points - the Endgame Labyrinth can be a complete bear and requires a certain level of competence in the game to even manage to buy a successful run through. The other three Lab stages, however - Normal, Cruel, and Merciless - are sort of standard equipment at this rate. The Labyrinth itself can be a good barometer of a build's Good-ness; if you can clear the Lab without too much issue for your difficulty, then you're doing fine.

ANYWAYS. before this gets even more verbose...hope that's some help to you! Good luck with relearning the game, and enjoy the final hurrah for Old PoE!