BIOSs differ, but if you are on the opening screen navigating instructions should be at the bottom, and a menu of options on the left. Select one (arrow keys), hit enter, and a new screen appears. The screen you want may be labelled "Advanced BIOS features" or similar.
However, it sounds like you have a disk failure/corruption or mobo failure and are trying to boot from another device (eg DVD). IIRC, the message you are getting means it could not find a bootable on any of the devices in its list, so you may also have to add a missing boot device. The option to do that would be on the same page.