What you want with the Junk folder is possible, but not exactly the way you envision it working.
Hold down the ⌘ (command) key, then press and release the A key, and release the ⌘ key. This selects all.
Hold down the ⌘ key, then press and release the ⌫ key, and release the ⌘ key. This deletes the selected entries.
The above sequence deletes the entire contents of the selected folder. Which could be your junk folder. Or whatever other folder you've selected.
The now-deleted contents of the selected folder are now (usually) moved to trash folder, and will be deleted per whatever rules you have set up for the trash. (The rules for emptying out the trash are on the same Preferences panel as the rules for the junk mail processing. You can set Mail.app to empty the trash on exit from Mail.app, or daily, weekly or monthly basis, or never.)
Then follow the preferences sequence from my previous reply to set up rules which delete the arriving messages — which can be spam and can also be real messages that got mis-classified as spam — automatically.
—— some general suggestions for your consideration...
For an introduction to using some of these key sequences and other details of the OS X operating system, see Apple's (free) Mac Basics web site.
If there's music notation software for iPad that meets your particular requirements, then that device might work better here. iPad devices feature less-complicated interfaces than does OS X.