The minimum essential educational qualification would be a high school diploma. The requirements vary depending upon the institutes. Some make it mandatory for the student to have some background in criminal justice before enrolling for their program.
This might mean previous education in the field or related work or even voluntary experience. But all online colleges or universities follow the general rule that you must have an associate level degree if you want to enroll for an advanced start bachelor level degree, that you must have a bachelor level degree if you want enroll for a master level degree and that you must have a master level degree if you want to enroll for a PhD level degree.
Apart from these general requirements, the only other credentials you will need are some familiarity with computers and the Internet. You don't have to be a geek but some familiarity will help. Most institutions offer a computer-orientation course to make you familiar with the institution's education delivery systems. Delivery systems and the different types of software or applications that various institutions use vary. Usually this is not a problem since each institution is supposed to make you familiar with whatever systems, software or applications they are using.
Jim Greenberg recommends you visit the Online Criminal Justice Degree Guide for more information on getting accepted to an online criminal justice university.