You should be able to execute the bootloader via hardware conditions at startup. Hold EA at logic hi and PSEN at logic lo during the falling edge of the reset pulse. This will force the bootloader to execute. Be aware that PSEN is normally an output pin. Thus, once the reset edge has fallen, it is probably a good idea to release the PSEN pin.

Once you've executed the bootloader software, the autobaud routine will execute. This means that the chip will sit there and wait for an uppercase 'U' to come over the serial input. Uppercase 'U' is just a sequence of alternating 1's and 0's, each one bit wide. This is how the µ-controller measures the baud rate.

All of this is explained in much more detail in the documentation you can download from Atmel.

Joe