How do I teach my dog to bark?

We got a puppy earlier this year; he is roughly 9 months. We are the third owners so I don't know his exact age.

Anyway, he never barks but we would like him to. We live on a farm just outside of town, and it would be great if he alerted us when someone was in the driveway. Additionally, crime is up in our area, and it would be fantastic if he barked at intruders. We don't want him to be vicious, just enough to let us know someone is there and to let potential intruders know we are aware they are there.

We've had him for 4 months. The only time he barks is at our gator tires. And once at a snake.

Sorry if this has been asked before, but most posts are trying to get dogs to STOP barking, but I'd like to teach my dog to START barking.