To enter your BIOS you need to press either DEL or one of the F keys when the computer is going through its POST (like the first 5-10 seconds after you hit the power button). This should bring you to a blue screen with grey and/or yellow writing. Im not familiar with Dell BIOSs so you will have to explore around the different options looking for it. If you think you screwed up some of the settings just exit the bios without saving or load optimized defaults and save/exit will fix anything youve changed.
Can I install the card first, then do this? Or do I need to change the BIOS first?