Embrace the grind and hit events like they are going out of style. That’s the spirit of the game.
Always be in a public team, because those convey bonuses and good teamsters share perk cards, even if it’s only a level one card. Other characters will make your character better, and can help boost xp gains. There are a small handful of stat checks in Wastelanders and the Steel series, so you can wait until you feel you have adequate stats to start the story missions. Consumables really help with these, since a few of the mentors and buffout variants can add +5 to certain stats. It’s not at all mandatory, but if you get OCD about that sorta thing, it’s just something to know upfront. On the flip side, the sooner you complete Wastelanders, the faster you start the reputation grind for the two factions (Settlers and Raiders).
Also, I would do the original story in small chunks, spaced out over a longer period of time. It’s pretty rough in spots, to the point that if you try to follow it all the way at through over the course of a weekend, not only will you barely level because the rewarded XP and items are laughably slight, but you might give up out of boredom. The story told is not horrible… but the gameplay which props it up, since it’s all told through holotapes, terminals, and such, is horribly, horribly bland. If you don’t pace yourself, following it exclusively could easily burn you out. It’s an incredible weak point of the game, which is unfortunately its introduction and breadcrumb trail given for you to follow out of the Vault. Wastelanders is a big improvement (though anything would be, hard to be worse) and most of Steel is better. If that pair has their weaknesses, they all feel tied to issues arising from Steel being split into 2 parts. Really hurt the pacing, but even with that in mind, it’s far better than base game. As much as 76 needs new content for current players, it needs to either revamp or cut the starting quests. It would be no real loss to cut most of the starter story and replace it. Too much of the original content is fetch quests with no stakes, no meaningful rewards, and no interesting hooks.
For leveling specifically, public events are great for this, because they are mostly popular, and you can participate without having to do the bulk of the work. People also often open lunch boxes for the ones where there is a high chance of enemy spawns and xp overload. Always go to Radiation Rumble. Always. Thanks to team share XP, you get credit for their kills. Focus on wounding enemies rather than killing to maximize XP and ammo efficiency, since you only need do damage to get XP and high level characters can finish them off. Some public events are worthless, like Open Range, which almost no one does and takes a long time with poor rewards. Always go to Scorched Earth. Smart players drop the nuke in such a way that no one has to enter the actual radiation area, so you’ll be fine. I would avoid the Earle mission. It’s very boring and can be a waste of time if you get a bad group, and not to be disparaging, but if other people see one of the limited slots filled by a low level character, they are not going to be happy. You can actually skip all of the story and hit the Enclave portion early, and those events are frequent, fast, and reward fair xp for completion and kills along the way. Expeditions and Daily Ops are also alright for XP, Decryption will be difficult for you but Uplink should be fine. Some mutations will be easier than others, but you’ll see that as you play. I’ve never done a low level trip through Expeditions, but the Pitt feeds pretty good XP. No idea if the general vibe of the player experience is to boot low levels or not. I’ve seen plenty of 2-3 man teams, so joining as a 3 or 4 should not be a hurdle. No time limit like Daily Ops have, so you’re not going to be taking the spot of a better character.
Surprisingly, the .45 pipe revolver, as either the rifle or handgun, is powerful and cheap to maintain, and might still benefit from being able to stack perks relevant to both weapon types. Until you start unlocking some of the better weapons or accessing decent or better legendaries, it’s a solid choice. The double barrel shotgun also shines at lower levels, but it tapers off as the enemies get spongier and by mid to high level, it’s not competing with Gauss shotguns, which have a greater effective range, or melee, which turns into silly high damage. One thing that’s very different from other Fallouts is that none of your gear matters unless you have a sentimental reason to keep it. Don’t think of it as normal gear hoarding, because it’s not. Leveled items are pure M(ini)MO. You will be constantly trading up until level 50, and legendary effects will keep that going until even higher levels, due to the RNG element of legendary rolls. You are likely to hit 70-80 before your gear starts to be stable.
TL;DR treating it like a M(ini)MO, and you’ll be fine.
