I have represented a number of people who have used a weapon- a knife or firearm to defend themselves. While injuries inflicted involved 'grievous bodily harm' I have not represented someone in respect to a murder.
Only use a gun or knife to defend yourself or family against a perception that you or they are about to be injured. I would not use a firearm to protect against damage to property.
After the deed is done contact the Police. Do exactly as they tell you to ensure that you do not wind up becoming a shooting victim yourself.
They may arrest you. Do not make a statement. Request to speak to a lawyer, request bail. If placed in the cells resist the temptation to discuss what happened with anyone else- other occupants of cells will often try and get you to talk with view to trying to do a deal with the Police to improve their position in respect to their own case.
Resist the temptation to yell abuse and vent. Remember your gaolers are probably taking notes.
The best way to find a lawyer is usually recommendation. This can be difficult if you are not of the 'criminal classes' and do not know anyone who can recommend someone to you. Talk to other lawyers and find out who is good in an area of practice. I would see who has recently given talks at continuing legal education training for lawyers and approach them. Often regional bars have referral mechanisms and if you explain the nature and seriousness of the matter they can arrange an appropriate referral.
Cases usually turn on the facts. It is therefore important if firearms were used that the Attorney understand how guns work. If he /she 'hates guns' they may not understand aspects of your defence.
I am a great fan of the Jury system. Unfortunately jurors include gun haters as well as owners. Try and use a type of gun that is common place and most people can understand you using- I would tend to choose a sporting firearm over a military firearm, and I would use common sporting ammunition.
I would seek to avoid doing anything that would lead to an inference being drawn that I had sought out or brought on the event.
Your position may be better if you used an improvised weapon such as a screwdriver to stab an intruder rather than a big knife.
I would avoid black semi automatic rifles, riot guns, firearms with silencers, ninja knives, rambo knives, knives with double sided blades that are clearly of an anti personnel rather than a hunting nature.