In NJ, the sentence would depend on whether the juvenile was tried in the Family Part (Juvenile) as a juvenile; or, was 'Waived' to be tried as an Adult. A juvenile that was charged and tried in Juvenile Court would likely face a four year sentence in a Juvenile facility. There are circumstances that could lead to either a longer or a shorter period of incarceration. My experience tells me however, that most cases alleging a sufficient probable cause basis to file such charges are 'waived', with the juvenile then subject to all of the penalties an adult would be exposed to for the same crime.
Under NJ law, a waiver of the case to adult court must be filed within thirty days of the arrest, or the case will be tried in juvenile court. Once the waiver is filed, a hearing is held at which the State has to establish probable cause to support the charges. Probable cause is a very low standard of proof - requiring only a reason to believe that a crime has been committed and that you committed it. Issues that are considered relevant to the 'waiver' include the type of crime; the manner in which the crime was committed; the extent of injury inflicted on the victim; the strength of the State's proofs; the prior arrest record of the person being charged as well as their prior conviction record (which are not necessarily the same thing); the prior sentences imposed for any prior convictions; and, all other facts that establish the crime.
Assuming the case is waived to adult court in NJ, for a 1st degree Murder you would face a thirty year sentence, serving thirty years of the sentence. Alternatively, the court could sentence you to a specific term of years between 30 years and life. A 2nd degree Manslaughter charge would result in a 10-30 year sentence. Obviously, if you can avoid the waiver, having the case decided in the juvenile courts would be the best alternative. I recommend that you speak personally with an experienced criminal defense lawyer familiar with this type of proceeding in the state in which you are located.