Just like in the tabletop version of Dungeons and Dragons, you can multiclass in Baldur's Gate 3.
Multiclassing in Baldur’s Gate 3 can be a little confusing initially, especially because it does not unlock until level 2. But, once players hit level 2, the option to multiclass is unlocked and is simple to set up. Be warned, your class will level up every other level. So, if you decide to multiclass, you will only put half as many levels in each class, giving you more options but fewer high-end abilities.
Once you hit level 2, the option to multiclass will appear after the usual splash screen, indicating what enhancements and options your new level brings. In the top left, near the “Level Up” text, you will see a small indicator of crossed weaponry with a “+” symbol. Scroll over it, and it should say “Add Class.” Clicking this option will allow you to start multiclassing.
From here, you can select a second class. Once again, be wary because you will be limited to leveling up that class every other level. This allows you to spread out your options but also limits your late-game abilities to some extent. Good luck, and have fun with your expanded options!