I'm looking through the PHB and I don't see any simple list of ways for a rogue to get advantage in combat. At high levels, a Rogue might have a special subclass such as Assassin that might allow advantage on attacks for sneak attack. However, if I were running a rogue I would make attacks without knowing whether I had advantage. At low levels, the only reliable way to make sneak attack seems to be to hide until the target is engaged with someone else and then stab the target in the back — that doesn't give advantage, but it appears to be the only reliable way for a low-level Rogue to sneak attack.
Am I reading the rules correctly?
Best Answer
Most of your sneak attacks will be triggered by an ally in melee with the target
You are reading the rules correctly: by far the most reliable way to trigger sneak attack is to attack an enemy engaged with an ally in melee. There's no need to hide or gain advantage: you can simply fight alongside an ally, or fight at range, shooting at the enemy your ally is attacking. As long as you have a melee attacker in your party, then they will be doing their best to always be in melee with at least one enemy, giving you a reliable trigger for sneak attack. This has been my experience playing in a party with a rogue, and watching/listening to D&D streams/podcasts with parties that include a rogue.
Of course, if you want to attack a specific target who isn't on the front lines and get sneak attack on that target, you're going to have a more difficult time. In that case, the next most reliable sneak attack trigger is probably to hide with your bonus action (Cunning Action, 2nd level) and then attack, gaining advantage from being unseen. Obviously this requires cover to hide behind, and you need to roll high on the stealth check, so it's less reliable than the "ally in melee" trigger, but it's still something that you can at least attempt on most combat turns, unlike most other ways of gaining advantage.
(Note: I'm not mentioning the optional flanking rules for gaining advantage, because if you're flanking then you already have sneak attack from an ally being in melee with your target.)