If you've been accused of first-degree murder, you're probably already working to devise a defense. Your attorney can help with this, and he or she will likely give you one of two options. Either you have a defense that you are innocent, or you admit the crime and argue that you didn't commit first-degree murder, but instead a lesser charge.
For example, if you killed someone in self defense, you may not choose to deny that you did, indeed, kill that person. However, the fact that you were fighting to protect your own life makes a major difference in your defense options.
If you decide to pursue this option, then you should be able to prove that you were defending yourself. This can sometimes be proven by medical reports and investigations that show the angle of wounds or attacks, impacts, or blood spatter patterns.
You may also argue that the prosecution doesn't have all the information it needs to prove that you intended to kill the person, and without premeditation, it's nearly impossible to prove a first-degree murder took place. You are not required to speak to the prosecution to defend yourself, especially if you would then be at risk of giving incriminating evidence unwillingly.
In cases where you want to prove that you're innocent, you may choose to claim that you've been mistaken for someone else. This could happen in cases where a witness or forensic evidence has turned up multiple possibilities or claims about who could be responsible for the murder of another person. If you can prove you weren't at the scene or didn't commit the murder, you may be able to get the murder charge dismissed.
Source: FindLaw, "First Degree Murder Defenses," accessed Oct. 08, 2015