Assassinate and Sneak Attack

Sneak Attack adds nd6 damage to an attack where you have Advantage on your attack, or the opponent is fighting at least one other person in melee. Note that the opponent fighting multiple people doesn't give you advantage, it just procs Sneak Attack. Sneak Attack can only be used once per round.

Assassinate gives you Advantage on an attack in the first turn of combat if you go before your target in the initiative order. This applies to every attack you make against them before their initiative.

As a separate effect, Assassinate gives you a guaranteed Critical Hit if the target is Surprised. Surprise is only ever the result of a Stealth vs Perception check before the first turn of combat, so this part of Assassinate will not proc unless you're starting the combat attacking from stealth.

Hiding during combat and attacking from Hidden (or staying hidden past the start of combat then attacking from Hidden) gives you Advantage on the attack, so will proc Sneak Attack, but the target cannot be Surprised past the first turn of combat so this will not proc Assassinate.