Ask HN: Best resources for home energy optimization?

I’ve been interested in home energy efficiency for years but I don’t have any experience with electrical work, HVAC, energy auditing, insulation, etc. I’m thinking about diving in one subject at a time as a hobby but I’m not really sure the best place to start?

Do I need to take a class to actually become an electrician to do the work on my house or are there good resources to self teach? Same with HVAC (I’ve heard self installing geothermal is possible)?