First, don't shoot anyone unless you have no other way to save your life. When you pull that trigger, you are changing your life forever, too.
Call 911 and report it immediately, then avoid getting shot by the responding officers. Preserve the scene (especially if there are other weapons present), and do what you can to get witnesses to stay or at least obtain contact info.
"It is, what it is": The real world is messy, and the law generally recognizes that. Getting caught in a lie or trying to "spin" the story is worse than having made a mistake, or being in a less than "perfect" scenario. Do not tamper with evidence or attempt to alter anything.
Make a common-sense statement to the authorities (suspect descriptions, what happened in general terms, etc.). After the basic statement, request an attorney be present before more questioning. Expect to go to the police station or even get arrested. Cooperate but don't get drawn into any games (good cop, bad cop, statements to avoid getting arrested, etc.). The detectives are experienced and you are no match for them. They are not your friends, but they are not your enemies, either. Be honest, but keep your mouth shut until you have discussed your case with your lawyer.
Try to get a quick call out to loved ones, and, without going into detail, tell them you've been involved in a shooting, that you're OK, and not to speak to the press. Tell them not to believe everything they hear in the media about the incident.
The rest you'll figure out as you go (statements, depositions, getting sued, etc.).
Last edited by R1145; February 18, 2013 at 04:25 AM.