You actually start a sentence with "You know," when you want to give someone information that you think they don't know.
Adding "You know," makes you sound intelligent and informed. But people can get annoyed if you use it too much or use it to talk about a subject that you don't have special knowledge of.
You wouldn't use "You know," this way in writing. When you pronounce this, you should stress the word "know":
You know, building muscle takes time.