When the computer starts you see that white text on a black page, with all the hardware and stuff (before the windows loading screen).
In the first screen (you have text with hd, cd drive ecc or an image of your motherboard) you have to press del or f1 (or other keys, depends on the motherboard) to enter the bios settings.
Usually it's written on the bottom of the first screen which key to press to enter bios settings (like "Press xx for settings).
Be careful to don't touch anything in BIOS settings if you don't know what you're doing (or at least don't save when you exit).
@slvrdrgn123: it's not always F10, sometimes it's DEL or F1 or other keys.