If your attic has no insulation right now then you're losing a lot of heat up through the attic and that makes your heating bills go up because the furnace or other heating source has to run more often to keep the house warm.
Insulating your attic can save as much as $600.00 per year and it's very cheap to insulate the attic yourself if you know how to lay some insulation or blow some insulation into the attic.
You should also find any air leaks around your house and seal off those air leaks where heating could be escaping and cold air could be getting into your house through.