With a bootable device connected, continue to tap on F12 when powering up the system.  You will see the option to select boot from one time boot menu.

you can only boot from a physical storage device that has a boot file on it, the device can be a ssd, hdd, dvd, or usb. 

before boot, pc needs to pass POST first:  BIOS performs a basic firmware test of pc that it needs to pass (Power on Test) before bios look for a bootable device to boot from.

you can enter bios by pressing F2 after power up, or press F12 then select set up from menu

to do a clean OS Install, boot from MS installation media on a USB or DVD.