When your PC first boots, you should see tons of text running through the screen. That's your POST. Right before that happens, somewhere on the screen it should say something like "press F8 for setup" or something like that. Press the key it tells you. After you press that button you should be in a screen with a ton of options and stuff. That's the BIOS screen.
But usually the time resets if the battery on your motherboard is out. If you look on your mother board, there's a button cell battery about the size of a quarter on there. Take the battery to radioshack and tell them you need another one of them. Then stick the new battery in there and you should be good.