That first night in camp with Astarion
The game doesn't really do a good job of explaining stealth mechanics. Advantage means attack rolls are more accurate, disadvantage means the opposite. Armor Class also affects this, so higher armored enemies are more difficult to hit. As you progress in the game this becomes a much bigger deal as martial classes (physical fighters) start to differentiate and gain certain skills.
Since you just started, you want Astarion to attack from behind in melee or completely out of sight if using a bow. If he's in the shadows even better. You can "force" this with use of the darkness spell/scrolls to limit enemy sight. You can use an offhand attack with your bonus action when dual wielding.
Personally, I think you have to do a little more "work" early on to make stealth characters worth it BEFORE your first major class pick. Just have fun figuring it out and later on you'll start experimenting and see how to make some truly killer stealth characters.