Mouse dragging detection

Hi mates, I'm making a game were the player has a spotlight and explores a few dungeons (in first person). I already made the player move but I actually have a doubt for camera rotating. I've been looking in a few forum but the issue wasn't exactly the same at mine and I'm new in Unity. The question is: How can I do to move my camera while dragging the mouse around the screen for the player to see what's happening around him?