(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. 

---

Cold Water Line

Turn off the cold water line that feeds the RO system, by turning the valve on the left side. 

Tank OFF

Tank is off when valve is perpendicular to the blue line. 

Tank ON

Tank is on when valve is aligned with the blue line.