I am assuming you are talking about an existing home. I would start with the easy, inexpensive things. Your energy is divided into two uses -- your baseload, which is your lighting, appliances, water heater, etc., and then your heating and cooling load. Heating and cooling typically make up about 50% of your energy. This is the area where you can make the most impact.
A good way to start is to get a good, comprehensive energy audit -- use a certified contractor from BPI or a similar program. Typically in an older home you will find the HVAC ducts, boots, and connections are leaky -- seal up these leaks with mastic. This can be checked by a duct blaster test.
Next, address your air leaks -- a blower door test can tell you how much air leakage you have. Start at the top in the attic air sealing -- seal around all plumbing and wiring penetrations, seal around the chimney, seal at all top plates, back up knee walls, etc. From there, move to the crawl space and, lastly, the exterior walls. Check weather-stripping on all windows and doors. Install an attic tent at your attic access and insulate the back of any attic doors. Only after air sealing, add more insulation.
Solar energy, geothermal and renewables are all great, but first concentrate on improving your thermal envelope. These are all low-cost, high-return items, and easy to perform. These measures should reduce your energy usage 20-40%. After these, you can move on to baseload improvements like appliances.
Be sure whoever performs the air sealing will take proper safety measures, check all combustion zones, and install a CO detector. Good luck.
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