If you're following Chicago note-bibliography format, you need to use footnotes instead of parenthetical citations. (Keep in mind that if you're using APA or MLA, you probably won't be using footnotes.)
Fortunately, Word makes this very easy!
At the end of the sentence where you want to add a footnote, just click the "Insert Footnote" button under the References tab.
Word will automatically place the correct number in your document. If you're going back and adding a footnote in between other two existing footnotes, it will also update all the numbers for you.
The cursor will jump down to the bottom of the page, where you'll provide the note in accordance with the style.
Note: The default formatting for footnotes in Word is not Chicago style! Footnotes need to be reformatted to size 12, Times New Roman with a first-line indent (like a paragraph), single-spaced (with one empty line in between each footnote).