A properly performed energy audit will provide you a detailed snapshot of where the opportunities are greatest to improve the energy-efficiency of your home. An energy-efficient home is an environmentally friendly home as well -- and one that will cost you substantially less to own and operate.
Energy audits cover everything from the amount of insulation in your walls, floors, attic, and roof; lighting; heating and cooling equipment; the distribution system that transfers heating and cooling; air leaks; etc. A proper energy audit will include a blower door test, which seeks air leaks in your home's thermal envelope); infrared cameras; and duct testing, for homes that use a duct system to distribute heating and/or cooling.
Contact your local utility provider, or providers, to find out if they offer no-cost or reduced-cost energy audit services. You can also contact local energy-services companies and/or energy contractors to find out whether they offer energy audits and what the cost of this service is likely to be. Do not underestimate the value an energy audit will provide to you -- you can't improve something unless you know where the issues and opportunities exist.