"Waste" one sheet of 8x10 film (or don't waste a good one and use one of your negs that you designate as "scrap,") take an empty holder, and find out what's going on. It' will probably take you less than 5 minutes to figure it out, and if it's something you need to do differently, it will take you 15 minutes of practice in the light to be completely comfortable with it and know it forever. Then practice 5 more minutes with your eyes closed. Voila!
You should have black border all around, and knowing no other details I would guess that you're not quite loading the film correctly. Are you getting it under those little runner things on each side? If not, you'll find that out someday when the film falls out inside your camera (I know from sad experience).
Anyway, play around a little with an empty holder and a scrap neg some evening while you watch "American Idol," and I'll bet you solve your own problem between commercial breaks.