Robert J
Depending in what form you want to create or compose:-
Musescore - traditional "sheet music"
https://musescore.org/en
Midieditor - for creating MIDI music files
https://sourceforge.net/projects/midieditor/
Tracktion - A full DAW, a recording studio in a software package
https://www.tracktion.com/products/t6-daw
All totally free.
inclusive_disjunction
Not really my thing, but LMMS is pretty popular.
https://lmms.io/