How do i delete a movie or video in the movie tab of my itunes

On your computer's iTunes you can select it and then press the delete key on your keyboard and that should delete it. If you are wanting to delete it from your iPhone then you should be able to delete it directly in the Videos app on it by sliding your finger across it on the selection screen in the app, and you should then get a Delete button on it.

If it's a download from iTunes then have you got a backup copy of it ? Films and tv shows can't be re-downloaded in all countries, and even where they can it's not guaranteed that they will remain available in the store.