It is good to hear that you have found a career to go into. Medical Assistant is good career choice. It can give you a good foundation if you decide to pursue Nursing school, which alot of people do including a neice of mine.
To become a Medical Assistant you need to contact a Community College that offers the program and speak to someone in admissions. They will tell you how to apply and the steps you need to take in order to enroll in the next available semester.
I pray all goes well for you in your new journey. Also, the Military also has similar jobs and they are really helping people to get in. This maybe an option for you since you are in job Corps. The military offers education, and housing all in one!