Your starting class in Baldur's Gate 3 determines your ability scores and what abilities your character will start the game off with. Each class can be fully leveled throughout 12 levels to reach its maximum potential, but all 12 levels don't need to be placed into only one class.
After receiving enough XP, you'll be able to level up. Leveling up will give you access to new abilities while also increasing your stats, but all of this is determined by which class you are leveling up. The game will automatically select the next level in your class, but that doesn't mean it's your only option.
How To Multiclass
Multiclassing can be done anytime with a level-up and has no restrictions. When you level up, look along the menu on the left. This menu shows the stats increased and new abilities being learned with this level up. In the top right corner of the left menu is the class icon. Select this icon to bring up a menu showing all available classes. From here, select which class you would like to level up in, multiclassing your character. Selecting a different class will change what that menu displays, now showing the bonuses for your newly selected class. Multiclassing can be done at any level with no requirements to choose another class, giving players the creativity to build their ideal character at any time.
Pros and Cons of Multiclassing
While there are no restrictions to multiclassing, some classes work better together than others. Multiclassing does mean you won't reach the full potential of either class, which can be slightly negative with certain classes. Spell-casting classes get access to higher-level spells towards the final few levels of the class, and multiclassing will lock off that caliber of spells, even if you multiclass into another spell-casting class.
While some classes don't work incredibly with multiclassing, others get some of their best abilities early on and are prime candidates for multiclassing. Classes like the Fighter learn Second Wind at level 1 and Action Surge at level 2, giving a multiclass with this class better long-term results. Before multiclassing, be sure to know what you might miss out on with your main class before unknowingly locking yourself off from powerful bonuses later on.