What is the most affordable way to improve the energy efficiency of old house?

I just bought my first home- a gorgeous old wooden character house built in 1910 located in central BC (northern Okanagan)! BUT, the average utilities bills total $410 per month ($258 for electricity and $152 for gas). The windows are old single pane and who knows what kind of insulation is in the walls. There is average insulation in the attic. The gas furnace is 18 years old and heats most of the house except the upstairs bachelor suite, which has electric baseboard heaters. The hot water heater was end of life and I will be replacing it immediately upon possession- I'm looking into the various rebates for a more energy efficient model.

So, I am looking at how I can save money on the bills without going into debt. I won't have hardly any money to spend and I won't have equity to borrow against for at least a couple years. 

3 windows and the glass on 2 doors are either cracked or totally blown out (they had some break-ins), so I was thinking of upgrading at least those to newer models. But I doubt that on it's own will do much. Another idea was to see if the gas furnace can be extended to heat the upstairs suite. And then the other idea I had was to insulate the exterior walls (and that's how I found this site), but that could be too expensive for me at this time. However, I was thinking of just insulating the exterior walls on the 2 dormers upstairs. They are missing flashing where they meet the roof, so we would need to remove the siding there anyways at some point to prevent further water damage. And since the biggest expense is electricity, I'm thinking the main culprit might be heating the upstairs.

Any ideas or comments would be greatly appreciated! 

-Ariella