NRA, first and foremost.
Your state rifle association, next.
There are several organizations/law firms who specialize in firearms defense and you can join, I forget the cost, not outrageous as I recollect, and it's like having an insurance policy. A little on line research will get your their contact information. Also, if your state has a concealed handgun association (Texas does) they will have the information. Many ranges, especially those that teach classes for CHL/CCW, have this information as well.
What to do and what not to do before your lawyer shows up is relatively simple. First and foremost, cooperate with the police. Part of that is, hopefully, getting your firearms out of action before they arrive. They'll like you better if you're not waving a gun around. If you are the one who calls 911 then you have to remember you're already using your mouth so use it wisely.
911 - What is your emergency?
You - A man tried to kill me/was going to kill me/I had to shoot him.
911 - [Assume they don't ask too many questions - if they do be prepared to answer them] - such as:
Are you armed?
You - yes but I put my gun down/away/whatever
911 - What is your location?
You - tell them as specifically as possible; an address is good. Intersections are good, too.
911 - Are you injured? Is anyone injured?
You - tell them if you are, tell them if the perp is alive or dead if you know, or just say the attacker has been shot and might need an ambulance.
Remember - cooperate, but say as little as possible while doing so.
911 - Help is on the way. Stay there.
You - Thank you. [And stay there!]
When the police show up they might ask for your weapon (likely they will). If it's on the ground or otherwise not attached to you tell them or show them. If it's on you tell them and let them take it. DO NOT TOUCH IT.
Police - What happened?
You - He attacked me; I thought he was going to kill me.
Police - did you shoot him?
You - I had to; I thought he was going to kill me. Can I speak to a lawyer, please?
No psychobabble, no excess yammering and details that don't matter, you're done.
Real world - it probably won't work out quite that easily, you'll be pumped full of adrenaline, shaking, maybe scared, maybe sick to your stomach (this ain't TV), just keep it to a minimum. Remember - cooperate. If they press for details of the attack don't tell them you won't tell them until your lawyer shows up. Just tell them the basics of the attack, you thought he was going to kill you, so you defended yourself. Don't be a jerk with the police. They're not after your hide; they need to file a report. Make their job as easy as possible while never saying anything stupid and remember you thought he was going to kill you.
Now you know what to do.
And yes, I am a lawyer, I am a CHL Instructor, and a Level III Firearms Instructor in Texas. The foregoing is not legal advice; you'll get that from your lawyer. It's just practical advice that I'd share with any of my students. Your own CHL instructor should have told you these things.
***GRJ***