Below-ground water shutoff question

The water line running into our house has a below-ground water shutoff about halfway between the house and the street. It is like an emergency shutoff I think. There is a cap that covers a cylinder with the shutoff. I need to use it because none of the shutoffs in our house work (it's a very old house, and the water slows down but doesn't fully shut off). In order to do this I will need a water key but then which way do I turn it and how much? The thing looks so old I don't want to force it the wrong way or too much and cause a leak in it. And what if I get a leak anyway, is there some plumbers trick to stop it?

Here's what it looks like: