First of all you need to find out the education institute or a law school that offers the degree of ACLU. Try to find the most reliable and well reputed institute and take admission in it.
However, before taking admission, make sure that you have passed the Law School Test (LSAT) as a good score in this test will help you in becoming eligible to be admitted in a law school.
Also make sure that the law schools you are applying for should be recognized by the American Bar Association (ABA).
Take the majors of criminal justice, pre-law and political science as these courses will help in having better understanding of the society and its laws.
If you want to practice law in a particular jurisdiction, then you have to take the bar exam comprising on many tests which will enhance your qualification and knowledge. You can also take these exams from your birth state.
After completing the degree, you should apply to join an established law firm or you can also start practicing privately. You can do it in both ways as there is no compulsion.
However, one thing is certain that you cannot make a huge amount as ACLU will earn you a supplemented income because ACLU usually works on volunteers.
Now that you have completed your education and have passed bar exams as well, go to the local ACLU and apply to become a lower for the legal division.