I'm suddenly responsible for people's lives and I'm terrified, what do I do?

The fear you feel is the right response to what you have been handed. People's lives now sit partly in your hands, and you understand that. Someone who took charge without a flicker of fear would scare me. Your terror means you grasp the stakes, and that is the first thing a good leader needs. You do not have to feel ready to be ready. No one feels equal to this at the start. What your people need is not a leader without fear but a leader who acts well in spite of it. You steady yourself, you make the call in front of you, and you keep learning. Competence grows from doing, not from waiting to feel calm. Right now, do one thing you can control: learn your people. Know their names, their strengths, what each one needs. That is where real leadership starts, and it is something you can do today. The terror will ease as you turn it into care and preparation. Let it make you careful, not frozen.