What is the best way to break up dialogue when writing?

I’m writing a novel and I have a question about he structure of dialogues. When writing dialogue between characters, is it necessary to indicate who is talking after each quoted piece if dialogue?

Or should it be briefly indicated at the start of each conversation? I’m concerted that if I was to literally have ,said John, quotation said Paul; ever line it would look too childlike and armature. And what if I have more than two characters in the one conversation?