Requirements will vary by jurisdiction. Most jurisdictions will require the applicant to hold either a bachelor's or associates degree in criminal justice, sociology, social work, or a related field of study.
State parole officers must have at least a bachelor degree, federal officers need at least a year of graduate course work. The degree can be from any accredited college.
Most parole officers started off as social workers at one time. Some community colleges offer courses in social work that lead to a certificate or degree in a very short period of time
I would think that studying Criminal justice would be a good start. Many universities have these types of programs. Junior colleges and technical schools may also have programs.