[hedora]

I would start by blowing insulation into the exterior walls (assuming they don’t have any), and insulating the attic. Also, upgrade the windows to double paned or better.

Fixing unintended holes in exterior walls is probably also reasonably safe.

However, randomly plugging intended leaks without understanding how the house is supposed to breathe can lead to condensation issues (mold and rot), carbon monoxide poisoning (if you have natural gas) and radon poisoning (if that is a thing where you live).

If you want to do more than I mentioned above, consider hiring an architect, or reading up on house ventilation designs.