Start with sealing all openings in the house to the outside, this would things like outlets, lights, pipes, and windows. You can get foam outlet covers that go under the plate and caulk where needed. That will prevent losing heat.
If you have access to wood but not a lot I would build a Rocket Mass Heater but with them you will need a place that can hold the wight. Search for "Erica Wisner RMH", the book on building them is
https://amzn.to/2lbIl9P