I acknowledge that Michael started to sing when he was 5, but don't know what was the first song he sang on stage with Jackson 5?

At age 5, MJ did not create any new songs yet. The first song recorded by Michael was "My Girl" by the temptations. Michael was known for his covering of music when he was a child. He did not start to become world known until age 10 when he sang "The love you save, I want you back, abc, ill be there, etc."