Efforts to insulate may be the ‘lowest hanging fruit’ of all.  Since heating and cooling are such big users of fossil-fuel based energy, keeing the heat/cool in or out as appropriate will save tons of CO2 emissions (and of tons money) in just a short time, usually paying for any costs within a year.  Such strategies only need be done once typically and the effects last for years. 

While much of what can be done with insulation may be more useful for builders and tradespeople, much of it can be implemented by homeowners and laymen.  So skip over what looks too difficult (or call a professional for help… it will pay off in the near and long term), and dive on into what you think you can handle.