The database table should be fairly obvious - you need three columns: a movie reference, a user reference (you will require people to log in to make comments, right?), and the comment.
The web page should be fairly simple also. Once the user logs in there is a comment field and a submit button. Above or below that would be the other comments. Most PHP tutorials start with something very similar - a page where you can enter data into a form and the data is displayed in a table on the same page. Same idea here.