An amendment is a change/modification to an existing document. The Bill of Rights, for example, are the first 10 amendments to the U.S. Constitution. Initially the Bill of Rights was not part of the Constitution; subsequent to those amendments, the Constitution was changed as per the amendments.
Slavery
In 1789, James Madison--nicknamed "the father of the Constitution"--proposed twelve amendments that ultimately became the ten amendments making up the U.S. Bill of Rights. In this respect, Madison was unquestionably the person who wrote the First Amendment.