what's a sentence for whistle?

He/She let out a low whistle.

I heard the train whistle as it came around the bend.

There's the song from the Disney movie, Snow White, "Whistle While you Work."

You'll get other ideas from the examples in this site.

http://dictionary.reference.com/browse/whistle

Before posting anything else, go and search up the terms you have, on a dictionary online. They have many examples of sentences; which after looking at, you should be able to come up with your own.

To form a sentence using the word "whistle," you can say: "I heard the sound of a whistle as the train approached the station."