What format do you want to use for your stars? text or graphics?
Asterisks can be used for a slightly easier implementation than graphics.
Define a text field for recording your star rating.
Define a global text field for specifying the rating when searching for movie records. You can set it with a value list of your star ratings.
Write a script that uses the value in your global field to find your movie records:
Enter Find mode 
Set Field [MovieList::StarRating ; "==" & Quote ( MovieList::gSelectedRating ) ]
Set Error Capture [on]
Perform Find 
Sort Records [no dialog; Restore ]
The == operator is the exact match operator and the quote function encloses the asterisks in quotation marks to keep filemaker from interpreting them as wild card operators.