I would remove the old insulation, carefully and bag it in large trash bags, because old rat feces and urine could be in it and the new insulation is so much more efficient and you do not want the airspace between the two.
How Insulate Attic Over Old Insulation?
Putting new insulations can be a very tiresome and dangerous task as it will result in contact with electrical wirings so you need to be very careful. You can use fiberglass batteries as it is the most common and cheap material available for insulation. For step by step details on How to insulate your attic see the link below:
www.thisoldhouse.com
www.thisoldhouse.com