Although the American Psychological Association and the Modern Language Association use different formats for many citations, both defer to "The Bluebook," the legal system of citation, when it comes to citing parts of the Constitution. The few modifications that APA and MLA make to the "Bluebook" citations when it comes to citing parts of the Constitution do not apply to the First Amendment because it is not divided into sections.
Decide whether you will mention the First Amendment in the text itself. If so, you do not need to cite it. For example, you might write "The First Amendment of the Constitution guarantees freedom of religion to all Americans" without including parenthetical documentation. If, alternatively, you write "The Founding Fathers guaranteed freedom of religion to all Americans," you would need to include parenthetical documentation.
Add the First Amendment to the reference list for your report, using this form: "U.S. Const. amend I."
Cite the amendment within the paper itself, in parenthetical documentation. For example, a sentence might read, "The Founding Fathers prohibited the making of any law that infringes on the freedom of the press" (U.S. Const. amend I).