Put gun away if possible then here are 5 things you should do including what to say....
1. Call 911
2. Officer this person attacked me, I will sign the complaint,
3. Officer here is the evidence (knife, gun, ball bat, whatever).
4. Officer these are the witnesses.
5. Officer you will have my full cooperation in 24 hrs after I see my attorney.
To be more specific..
1. Call 9-1-1. Ask for an ambulance and police. Asking for an ambulance first then police shows the human side of you.
2. Tell the operator "I shot a man that was trying to hurt me". The key words are shot (not killed, off'ed, smoked, etc), man (not a teenager, child, etc).
3. Give your location and describe yourself. Most likely your description will be relayed to the first responding police officer. It would be to your advantage if the officer has an idea of who is the "good guy".
4. Do not answer the operator's questions for details. You are being recorded so you have to be brief. Say that you are really busy right now but you are willing to stay on the line until police arrive.
5. Speak clearly. This projects the image of being in control and not panic.
6. Do not have your pistol in your hand when the officer arrives. Holster it when you see the car approach. Keep your hands visible from this point forward.
7. Give the officer a brief non-emotional description of the events. Give enough to make the initial police report. He/she needs basic information to decide if you are free to leave, getting a free ride to the police station for more questions, or getting the handcuffs.
8. Avoid giving out too much information. Your emotions will come out and may work against you. Say that you are trying your hardest not to throw up right now and would rather answer questions later. Police officers who have been involved in a shooting will sympathize with this. If you did #7 well, this should not be a problem.
9. Others. Don't argue with witnesses. Don't act aggressively even if you're angry. Move slowly.