When I open a new drawing, it's always a few existing layers there, that I haven't made. I can not delete them, but I can change name and colour and stuff like that. What are they there for? Should I leave them as they are, or can I just change them as I like?

Maybe someone in your company added the new layers to the default template you are using.The template files that come with AutoCAD generally have only one layer named "0".Yes, you can leave the layers but I wouldn't change them without first making sure it is OK with your boss.