Multiclassing

Multiclassing lets you gain a level in a different class than your starting class. The only requirement is that the main ability of your first and new class must be 13 or higher, indicating some sort of skill at it. If you multiclass, your proficiency bonus is based on the total charcter level.
For magic wielders, the spells known derive from the spell pool for each class, and are just added together. To determine your spell slots, the amount of levels you have in classes are weighted differently: Bard, cleric, druid, sorcerer, warlock and wizard classes are counted normal. Paladin and ranger count as half. Fighter (Eldritch Knight) and Rogue (Arcane Trickster) count as a third. Round down the result and consult this table: