The “new song” we sing does not have to be a newly composed worship number. The new song is merely a fresh response to praise and thanks - one that matches the freshness of God’s goodness and mercy, which are “new every morning” (Lam 3:23). A new song springs forth unrehearsed from the heart of a worshiper who has been struck anew with wonder at the greatness of God and the salvation He has provided. When we see the mighty hand of God working in a way we’ve never observed before, we can’t help but burst forth with a song we’ve never sung before.
https://www.gotquestions.org/sing-to-the-Lord-a-new-song.html