I'm looking for the name of an older movie about a girl in a dark blue velvet dress who was killed I think by her father and buried in the garden.. I think her name is sarah and she haunts the girl dating her boyfriend I know it has shannon dortzy in it.

There were no hits at all on a Shannon Dortzy, so if you mean Shannen Doherty,
There's a link to most of the movies she has been in at the link below:

Shannen Doherty Filmography (http://www.fandango.com/shannendoherty/filmography/p19482?sort=release-date&srtste=sorted_up)

From your description, it may be: Friends 'til the End Synopsis (http://www.fandango.com/friendstiltheend_v412176/summary?sort=release-date&srtste=sorted_up)

Sounds like Buried Secrets (1996) (TV) (http://www.imdb.com/title/tt0115788/) with Tiffani Thiessen.