Which OS do you have installed, Lion or Mountain Lion? Go to System Preferences->General. There will either be an option to not restore windows when reopening applications or Close windows when quitting an application. Enabling this will affect all applications. Alternatively, you can hold shift when launching Safari to prevent it from restoring windows. If your want a blank page to load and it doesn't after this, clear out the address in the Home Page field of Safari preferences->General.