With the 595 you clock data into the serial registers with the clock and data lines. The output pins will not change until you clock the RCK pin.
To do as you wish you would,
Momentarily put 0 on the /SCLR pin to clear the serial registers or put zero on SI and send 8 clock pulses to SCK.
If you now send a pulse to RCK all the outputs will go to zero (if /OE = 0).
Put a 1 on SI and clock SCK to place 1 in the first shift register.
Pulse RCK to transfer the serial registers to the output - QA will go high.
Repeating the last two lines will make the next bit go high.
This is all explained in the data sheet.
Mike.