As it's an embedded platform, the Raspberry Pi doesn't have a BIOS like you'd find on a conventional PC. The various system configuration parameters, which would traditionally be edited and stored using a BIOS, are stored in an optional text file named config.txt. This is read by the GPU before the ARM CPU (and Linux) is initialised; therefore it must be located on the first (boot) partition of your SD card, alongside bootcode.bin and start.elf. This file is normally accessible as /boot/config.txt from Linux and must be edited as root; but from Windows or OS X it is seen as a file in the only accessible part of the card. If you need to apply some of the config settings below, but you don't have a config.txt on your boot partition yet, then simply create it as a new text file.