How to write a program which can run on a computer without a operating system?

If there is a computer without operating system, then we can install one on it using a floppy disc or usb stick. While the computer is booting, we push F11 or some other button, then the computer will boot from the usb stick and run the system installing program. Now my question is that: How to write a program into the usb stick such that if we press the F11 button, the screen will print "Hello World!" or show some pictures.

How to realize such an program using C and assembly language? How to write such program into a usb stick?

Thaks