First, how can i put the facebook comment form above wordpress comment form. In default, i saw the facebook comment form locate below wordpress comment form.
If you want to remove the place of the facebook comments you must disable the auto insert of comments, and add it by shortcode on your templates. Read this topic for more info
And second question, how can i centralize the facebook comment form because i saw its at left alignment.
the comments box is added inside a div container.
You can manage the Css of this div to feet your need.
margin: 0 auto;
width: 300px; /*should match with the comments box width*/