It can sometimes be beneficial to choose a class like Fighter or Rogue at level 1 for their excellent martial/skill proficiencies, even if they aren't going to be your main class focus. Proficiencies: Classes gain fewer weapon, skill, and saving throw proficiencies when chosen later on than at level 1.A Wizard 4/Cleric 3 can only cast level 2 spells from each class, but will have a large number of spell slots to devote to each, with their Cleric spells able to go in Wizard slots and vice versa. Additionally, spellcaster/spellcaster multiclasses have a shared pool of spell slots, removing a major disadvantage of this kind of character in older versions of D&D. Spell levels: Full casters gain new spell levels and spell slots every odd class level.Most spellcasters, and also Paladins, choose their subclass at level 1. This is often, but not always, the minimum number of levels you want to hit in a second or third class.
Subclass: Most classes let you choose a subclass at level 3 (your Bard college, for example).Extra Attack is a priority ability you don't want to delay more than one or two levels. Pact of the Blade Warlocks get a special Blade Pact Extra Attack at level 5 that does stack with the normal Extra Attack (usually it's just one and done).
