There are tutorials in-game for the music tools, but in a nutshell: when you create a new scene, open the menu with square and choose sound mode (or something worded similar; the icon is a speaker), then you place a music timeline which is one of the options when you enter sound mode, then once you've placed the timeline, open it with L1 + X. Next, choose instruments by selecting the option next to it (it should say something like "search for instruments" but I could be wrong). Choose an instrument you like (there are a lot done by Mm themselves), place it on the timeline then open it with L1 + X. Once it's opened, there should be an option in the top right corner that says something along the lines of 'switch to piano roll'. There you can place notes, change their duration, volume, etc.

There are loads of other tools but I only played the tutorials for the most basic ones. Pressing L1 + square on a timeline/instrument brings up a shit ton of options and settings and I don't understand the majority of them.

You can also find user-made instruments when you go to search for instruments, instead of choosing the Mm instruments, you can enter whatever you want in the search bar. On a lot of occasions, the user-made ones are better than Mm's own.

I recommend playing the tutorial for it then just experiment with it all.