A lot of the companions in Baldur's Gate 3 will appear in the first couple of hours of the game, but Karlach is one of those rare companions that might take you longer to find. In order to get to her, you need to make your way over to the Goblin Camp where you're directed towards by the Druid Encampment.
On your way there, you'll go through an area called 'The Blighted Village'. Don't take the exit towards the Goblin Camp, instead take the only other exit. This will put you on the path toward Karlach.
You'll have to battle some Gnolls to get there, but once you're in the village past that you meet some Paladins who will send you to where Karlach is. Without spoiling anything about the story or her questline, this will trigger a quest that you have to complete in order for her to join your team.
Now you've got a Barbarian as part of your crew! This class is really good at tanking hits and also absolutely dishing out the damage, mainly using melee weapons rather than any form of magic. The sheer amount of damage she can dish out will make her your main melee damage dealer in absolutely no time at all.
