This is almost impossible to answer because Criminal Justice is a huge field. Basically all court cases are either criminal or civil. While Civil is somewhat bigger, criminal justice makes up a major part of the justice system.
Second even if you brake the lawyers who work in the criminal justice system down to the most basic job, you are still left with at least two: Criminal defense attorneies and Criminal prosectioning attory.
Defense attorneies: Defend the people that the police arrest for crimes.
The Prosecution: tries to prove that the police arrested the right person, beyond all reasonable doubt.
But again this is the extermly most simple terms and discribtion of what they do. There is an extreme difference between what a defense lawyer in traffic court does vs. what a defense lawyer in fedural court or on a murder trial does.
But to your question about what is nessecary if you are in high school and looking to work in that field. Really nothing. You still have to go to college, your grades and LSAT (the test for law school) are the most important thing. Even what you major in highschool does not matter that much as long as it is a serious course. English, History are among the best because you have to do a lot of writing which law schools like. But they really don't care that much about your major in college (unless it is something where there is very little writting like Art or Gym) and law school don't care about high school at all. You can be the worst HS student if your college GPA is 3.8-4.0 and LSAT is 175-180 law schools will be all over you.
To become a divorce lawyer you need to go to school and get your law degree. To be involved in family court you need to go to family court and get involved.
Let me ask you a question that may help.
How does anyone get involved in anything?
Depending on what aspect of family court you want to get involved with and to what degree, the amount of training and or schooling will vary.
You could become a janitor and clean the courtroom with little or no training but if you want to become a lawyer you need to start with basic 101 college stuff. While in college they will let you know what else you need to do.