When your computer starts up, there should be a screen like a welcome screen that most likely says "Gateway" on it... There should be a message somewhere that tells you to press a key to enter "configuration" or something like that. Most likely, that is the BIOS. Look around closely though! On mine, since I custom build my PC, says "Asus" and in white lettering on a white background it says "Press delete to enter setup", and I find it really hard to see. It took me a while to find the writing.
Hope that helps.
-Jamxx