I had to do it for the place I'm working now around a year and a half ago. Just go to this site:
http://www.jus.gob.a...es-penales.aspx as they explain everything there.
In my case I made an appointment (I believe it was for the same day, they have a really open schedule) for the place in Tucumán 1353. This is the second time I did this, so they already had my information on the system and it was a little faster. You have to sign a few things, they will take your fingerprints, but the whole process doesn't take much. Right now is costing 40$ pesos and they give you the certificate in 5 business days, or you can rush that paying more but I don't think is necessary unless you really need it fast. You will need to go a week later for the certificate.
I'm checking the site, the next appointment is on thursday. You can choose 10,30am, 11, 11.30, 12, etcetera. It's easy and fast, don't worry.