Tiffany - I think I can answer your question.
This is one of the hardest things for newcomers onto BFL - trying to determine what amount of weight to start with. Start LIGHT with the weights and work on FORM and cadence (the speed at which you lift and lower the weight - you want to keep it steady and slow like Bill says in the book).
If walking up a flight of stairs does you in, don't compare yourself to someone at the gym who's doing 8-minute miles on the treadmill - you're not there yet! But if you keep doing the 20 minute interval training that is recommended in the book, by the end of the 12 weeks your '10' will be much higher - maybe running up the stairs! Believe me, I know how it feels getting blown out just walking up a flight of stairs - I was there once...
Back to your weights. Are you working out at a gym? Hopefully they have a good set of dumbbells, preferably starting at 2.5 or 5 pounds and going up in 2.5 lb increments. For arm and shoulder exercises - start with the 2.5 or 5 pounds for your first set of 12. At the end of the first 12 reps (done with strict FORM, of course), if you feel you can still do more reps at that weight, then move the weight up another 2.5 or 5 pounds per dumbbell. If you can't even FINISH 12 reps, then stay with that weight for the next set. For chest, you may want to start a bit higher - maybe with 10 pound dumbbells. If you do the splits and rests between sets the way the book says, and use FORM, then you'll be amazed at how quickly you progress!
Hope that helps. Feel free to join us at the BFL thread!