How can I teach a grown dog to sit?

Hey there fellow dog owners. I adopted my dog a few months ago when he was already five years old.(human years) He's probably the most adorable dog I've ever met and loves affection, though, the older owners have never taught him to sit or do anything else, other than coming when you call him. Is there any way that I can teach him to sit, now that he's a grown dog? Will it be hard? I'm talking about a male yorkshire terrier.

Thank you in advance, it'd be really awesome if I can teach him.