When you send stuff to a shift register with shiftOut you send a variable containing a number. This is held in the computer as a binary number. To turn all the LEDs off you might set this number to zero.
To turn only the last LED in the shift register on the number you send hasto have a logic one in the lastbinary bit position in the word. So to do this you can use the set bit for bit seven instruction on the variable that was previously zero.
Look up the syntax of this instruction in the referance section.