There are two ways to end a line of dialogue that isn’t meant to stand as a complete sentence. One is with a dash, the other is with ellipses (those three spaced dots, remember?)
They aren’t interchangeable.
Ellipses are for utterances that trail off in some manner:
“Well,” she said, “if that’s what you really want . . . .” (That’s the ellipses, plus a period.)
“Well . . . if that’s what you really want, I suppose it’ll have to do.” (That’s just the ellipses, showing how the speaker lets his or her voice trail off into a significant pause before going on to the rest of the sentence.
Dashes are for utterances that are broken off or are interrupted:
“I told you I wanted–”
“I know what you told me, but the store was all out of them.”
“And the winner is–”
“Anastasia Oddfellow of East Drumstick, New Jersey!”
Got it? Good.