I got a new MacBook replacing an old one with iTunes. How do I get iTunes on my new computer

It should come pre-installed.

Note that recently, iTunes was renamed 'Music', so if your previous machine was running an older OS version you might not have seen the name change.

The name change was, in part, to reflect the fact that it focusses on music and other non-music tasks that the iTunes app used to perform (such as managing your iPhone) has been moved elsewhere.

This article might help:

https://discussions.apple.com/docs/DOC-10831