How to close this water valve
Those are called stop valves; when configured with the outlet(s) at 90° from the inlet they are commonly called angle stop valves. Yours look like dual outlet stop valves. To shut them off you turn the oval shaped knob clockwise all the way.

Internally, your valves use a bibb washer and seat assembly to effect shut off. It is very common for the bibb washer to be so soft/worn that tight shut-off is no longer possible. You could try tightening the knob with pliers, thereby forcing the washer (or whatever remains of the washer, or maybe the end of the stem where the washer used to be) onto the seat, to try to get the water to stop flowing.

In the end, if tightening the knob clockwise does not stop the flow then the valves need to be replaced. Many are poorly/cheaply made. Make sure you buy high quality replacements (not cheap ones) and consider the ball valve style, rather than the bibb style.