I thought you only italicize a title of a book, music etc.. I'm not honestly sure about If you do so, what words must you add and where must you add them?
You can italicize a part of a quote to draw emphasis, and indicate that the italics were added and not part of the original quote in brackets.
"I cannot say with certainty that it wasn't me. [emphasis added]"
Different style guides differ on exactly what words to use and where to put them, but the MLA style guide offers a good example:
Emphasis may be added to a word or phrase in a quotation by placing it in italics. When this is done the note [emphasis added] or [italics added] must be inserted in brackets at the end of the quotation (within the quotation marks), or if the emphasis comes at the end of the sentence, in parentheses outside the quotation marks.