What is the song that goes when i was a young boy at the age of five my mother said i will be the greatest man alive?

The song you're referring to is "Welcome to the Black Parade" by My Chemical Romance. It opens with the lyrics, "When I was a young boy, my father took me into the city to see a marching band." The song is a central track from their 2006 album "The Black Parade" and is known for its themes of loss, hope, and the journey of growing up.