Getting started with greening an existing home will require a home energy audit. An audit can assess how much energy your home consumes and identify what measures you can take to make your home more energy efficient and show ways to conserve hot water and electricity. An audit will show you problems that may, when corrected, save you money over time. During the audit, you can pinpoint where your house is losing energy.
USDOE's Consumer Guide to Energy Efficiency and Renewable Energy (link below) gives both a good DIY checklist and recommendations for a more thorough professional home energy audit.