in short, when the valve handle is in line with the pipe, the valve is open; when you turn the handle 90 degrees (1/4 turn) the valve is closed. Usually, there is a metal tab sticking up next to the handle to prevent the handle from turning more than 90 degrees. In your case, it looks like you should turn the valve to the left (as you are facing the wall), to close it.
Sometimes, they require a little oommph to get started, but as long as the pipes are in good shapes you can give it a little oommph without damaging the valve. A ball valve is generally the better kind of valve to have, so that's a good thing.