A simple way to start writing music is by downloading a free music composer program. Such a program could be MuseScore, is free and it is pretty simple to use. Someone has to know the basis music notes to be able to write music. That notes are nto, re, mi, fa, sol, la, si, nto, in my language or A,B,C,D,E,F,G in other languages. Someone of course can start writing music without knowing the notes by simply adding the notes into the program and getting a nice or noisy sound at the end.
What I have done is to first download the program and then write up some notes in the way you want them to sound by saying them out-loud. Then put the notes into the program and play it.. see what you can get. Good luck.