Bios is the firmware of the motherboard and allows an advanced user to tweak settings to improve performance or stability. All motherboards bios is at default auto-detect settings when bought, so theres nothing that needs doing besides changing the boot order for your hdd's, cdrom's etc.
Press delete to enter bios just as the computer is powered on, may need to wait a couple seconds but repeatingly hitting delete will help.
After the computer is built, the next step is installing the operating system. If the operating system is off a CD, will need to enter bios and change the boot priority so the system knows to load and install from the CD. After operating system has finished installing, change the boot priority to the hdd it was...