5e Sneak Attack rules are also pretty simple: are you wielding a finesse or ranged weapon, and do you have advantage on the attack or a non-incapacitated ally within 5 ft. of your target (or, technically, an enemy of the target) and don't have disadvantage on the attack? Have you not yet used Sneak Attack on this turn (it can be yours or somebody else's if there's something allowing you to attack outside of your turns, like an opportunity attack)? Then you can Sneak Attack.
There isn't anything restricting the types of creatures you can Sneak Attack (presumably you are hitting some sort of structural weakness or something) or a limit to how far away ranged weapons can be used. Technically, you'd probably be limited to the normal range of weapons, because you have disadvantage to attack at long range, which would cancel out any advantage you would have and nullify the effect of having an ally next to the target, but, if you have the Sharpshooter feat, you can ignore disadvantage from shooting at long range (so, if you have some sort of build that gives you the Sharpshooter feat and also something that allows your rogue to use longbows (such as being an elf), you could potentially Sneak Attack people from up to 600 ft. away, though you'll still need an ally next to your target or some source of advantage).
So, probably the most reliable way to get Sneak Attack is to just have one of your allies standing right next to your target. The next is attacking from hiding, as this gives you advantage for attacking a creature that doesn't see you (this doesn't just apply to rogues; anyone gets advantage for attacking a target that can't see you).
For this, you'll need something to hide behind, and have to take the Hide action to properly hide yourself (which rogues of 2nd level or higher can do as a bonus action). This requires a Stealth roll, which is compared to the creature's passive perception, to work, and after you make the attack, whether you hit or miss, the creature can see you (so you'll need to hide again to use this tactic).
There are also other ways to get advantage, such as having someone cast Faerie Fire or Guiding Bolt (both 1st level spells), having a familiar with flyby (ignores opportunity attacks) use the Help action to distract them (granting advantage on the next attack against them), blinding or stunning them, knocking them prone (only really helpful if attacking from melee, and I'm not sure how a rogue would do it alone; maybe pick up the Shield Master feat (and also shield proficiency) and make sure your DM is okay with you using the bonus action shove (to knock them prone) before you use the Attack action).
As far as feats, there's Grappler, which gives you advantage to attack creatures you are grappling. It takes an action to grapple (or, technically, part of an Attack action, but that's immaterial since rogues never get Extra Attack), but it also takes an action to try to break out of a grapple, and, if you use Expertise as a rogue to double your proficiency with Athletics, it can be very hard for them to break out of it. Only works on things up to one size larger than you, though.
There's also Mounted Combat, which lets you roll with advantage on melee attacks against any unmounted creature that's smaller than your mount. So it'll probably take DM approval to get anything larger than a horse or possibly an elephant, and you're limited to only finesse weapons, but you can use this to do ride-by Sneak Attacks.
These are just a few ways to get Sneak Attack; the rules for it are lenient enough that all kinds of weird combos and builds will work. Now, if you excuse me, I'm off to build a cavalry rogue.