Go into the "Mail & News Preferences" and then click on the "Reply & Forward" tab. In the "Reply & Forward" section at the top, you can change the message quoting with the top two options.
In the "Mail Attribution" section in the middle, click on the second bullet option ("Prefix my replies with Internet-style attribution lines") and then either select or deselect "Place insertion point before quoted text" depending on your preference. Checked will quote the message in the reply and place the cursor at the top of the message so you can begin typing on top of the quoted text; unchecked will place the cursor below the quoted text.