You can start selling your music right away once you become a Noteflight Premium subscriber and set up your seller account with a US bank account. Once you've subscribed to Premium, sign up to become a seller here: https://www.noteflight.com/account#/selling_setup.
You can see which songs are permitted for sale by checking titles in our song catalog here: https://www.noteflight.com/marketplace/song_catalog
If you have any questions, you can review the Seller Guidelines here: https://www.noteflight.com/marketplace-selling-and-purchasing.
If you haven't been a Noteflight user before now, you'll need to import your scores into Noteflight so that you can sell your arrangements and compositions as Noteflight scores. The easiest way to do this is to click "Create" in your Noteflight account and import from XML from whichever notation file you used. Make sure everything looks right then click "Score Details" to sell your score!