If the party starts combat when one player acts, I tend to rule that's when we begin the turn order.
Everyone is ready to attack and the fighter kicks in the door. Roll initiative. The fighter gets a 12. Characters could act before the fighter, but they weren't the one that started things, and it makes zero sense that they're able to do anything before the fighter's action occurs. And it also makes zero sense that the fighter, that started the fight and initiated action, might act in the middle.
If a player has something they really want to do first, I'll ignore the above somewhat. "You're fast and can see the fighter about to act, and have time to cast bless before they can finish their thought." Because that's more fun for them.
It can get funky if everyone is aware of each other. Most often it makes sense to begin with the person that started things. I could say everyone saw them about to act, and began striking preemptively, but that 's often funky. Especially if there's a lot of combatants who shouldn't have been paying that much attention. And if they instigator rolls ass for initiative.