(If your system is leaking, the first thing you will want to do, is turn off the cold water line, which feeds the system. (Red circle on Left).
The second thing is to turn off the tank valve. See pics below for on and off positions. (Red Circle on R.)
Then (or before) call USPW 415-883-9900. We can walk you through some troubleshooting or can schedule a technician to come take a look and fix the leak.