Should I learn old Objective-C memory management or skip it and use?
You should learn how memory management works whether or nor you use ARC, and regardless of what operating systems you need to support. It's important to understand what's actually happening with memory in an app even if you don't end up writing the code that manages it. ARC can do most of the work of this but if you don't understand what's going on then you might as well be trusting in magic invisible fairies to clean up after you. Don't trust in magic, learn what's really going on.