Aero barrier and/or spray foam for an existing home should work well. But they are not long lasting and are often not environmentally friendly. You can also look into exterior insulation.
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.
Some home energy companies will do a free audit, in my case it's Mass Save. My in-laws on a limited income even got new windows and insulation for free.
Do a blower door test and see where the air is coming in/out:
* https://www.energy.gov/energysaver/blower-door-tests
* https://www.youtube.com/watch?v=msZ_E-4GFs8
* https://www.youtube.com/watch?v=Q5vHxgq7GKo
Then plug the holes.