There are no minimum qualifications for ACLS other than making sure your BLS is current. In ACLS you will be tested on the BLS survey. You also need to be comfortable with rhythm interpretation as that is not really the main focus of the ACLS course. If you are uncomfortable with rhythm interpretation, I suggest ECG Workout by Jane Huff RN which is a very informative book and offers a lot of practice.
I suggest taking the course in a hospital as opposed to online. I think having an instructor is beneficial especially the first time around. Also, having ACLS prior to having a job is beneficial, but most employers require ACLS within a certain period of hire (such as within 6 months of employment or something) and most employers also pay for the course instead of having the cost come out of your own pocket. Just something to think about. Good luck!