Was wondering about how you can use a sneak attack with dual wield: if you cast true strike (gives you advantage, say you know it because you’re a high elf), can you then take the dual wield bonus action? If so, does this give you the ability to make a sneak attack? I’m trying to think of ways to get sneak attack on every turn and this seems like the simplest.

The two-weapon fighting bonus action attack is only available to you when you take the Attack action.

Also, True Strike’s advantage doesn’t take effect until your next turn.

True Strike is a trap, don’t go for that. As a High-Elf Rogue, just take Booming Blade and Disengage backwards to force your enemies to move after you and take the retributive damage.

Yeah, that doesn't work for all the reasons mentioned above. True strike is bad.

The most consistent way of getting sneak attack is to have a friend in melee. The second most consistent way is to attack from stealth, then re-hide and move (a bit dependant on terrain).

You could always take 2 levels of Barbarian for free advantage through Reckless Attack. It requires you to build for STR rather than DEX (you can use STR with finesse weapons), and at first it relies pretty heavily on Rage to keep you alive, but as you keep leveling Rogue and get things like Uncanny Dodge and Evasion it gets a little safer. Danger Sense also somewhat makes up for your lower DEX save. It's a pretty interesting build.

Or just be an Arcane Trickster at level 3 and pick Find Familiar as your exception spell. Make your familiar always use the help action to you and always attack with advantage.

Quote from githbrains >>

I’m trying to think of ways to get sneak attack on every turn

 You and every rogue out there :)

Quote from githbrains >>

Was wondering about how you can use a sneak attack with dual wield: if you cast true strike (gives you advantage, say you know it because you’re a high elf), can you then take the dual wield bonus action? If so, does this give you the ability to make a sneak attack? I’m trying to think of ways to get sneak attack on every turn and this seems like the simplest.

No, as everyone has pointed out, that doesn't work. I'll summarize the key interactions:

1. Two-Weapon Fighting
   • Requires the use of the Attack action
   • Casting True Strike (or any spell with a casting time of 1 action) requires the use of the [Tooltip Not Found] action
   • Both actions are mutually-exclusive on your turn in nearly all circumstances
     • Taking both actions on your turn can be accomplished if you have Action Surge from levels in the Fighter class, or if you are under the effect of the Haste spell
2. Sneak Attack
   • Requires a ranged or Finesse weapon, and one of two qualifying circumstances
     • Advantage on the attack roll
     • An enemy of the target (doesn't have to be your ally) within 5 feet of it
       • The enemy of your target cannot be Incapacitated
       • You cannot have disadvantage on your attack roll

Since you're already talking about casting spells, I'm assuming you're an Arcane Trickster or a caster that's dipping into Rogue? Shadow Blade is perfect for this. It's a bonus action to cast, so you can take your Attack action on the same turn. The summoned weapon has the Finesse and Light properties, so it is fully eligible to be used with both TWF and SA. When attacking a target in dim light (very common) or darkness, you automatically get advantage with that weapon. As icing on the cake, attacks with the weapon deal more base damage than any base weapon available by RAW, and the damage scales if you upcast. Total package.

In addition to what was said above, the reason that True Strike is a trap and won't work, even when you can cast a spell and make a weapon attack in the same turn, is that True Strike does not give advantage to your next attack like you would think, it gives advantage to your first attack on your next turn against that specific target, and requires you to maintain Concentration the whole while. 

Excellent. Thanks for the help, everyone! 

Just to talk about True Strike a bit... as a gag I had the players in a curio shop where they could buy some magic items, and the owner had a big stack of scrolls of True Strike that he's desperately trying to get rid of... basically everyone in the party got a True Strike scroll as a 'free gift" for any of their purchases.

Quote from TransmorpherDDS >>

Just to talk about True Strike a bit... as a gag I had the players in a curio shop where they could buy some magic items, and the owner had a big stack of scrolls of True Strike that he's desperately trying to get rid of... basically everyone in the party got a True Strike scroll as a 'free gift" for any of their purchases.

 I am stealing this. This is will be a thing in my world at some point now. Thank you so hard, from the bottom of my glee.