I'd purchase a couple of nursing tanks and wear them under your regular shirts. I am pretty sure they sell them at Walmart and Target, but if not, you can get them at Motherhood Maternity.... I believe they usually run around $10-$15.... and if you don't have a Motherhood nearby, their online store is pretty efficient.
Nursing tanks will give you the discreetness you desire, without having to spend a fortune on nursing tops. Then if you find that you are doing well with the nursing, you might invest in a couple of nursing tops later on (I bought a couple of nicer ones to wear to church and for going out). BUT, don't buy the tanks until after your milk comes in and you know how big your boobs are going to get (unless there is a one size fits all). My boobs jump from an NA size, to a FF size when I'm pregnant and nursing!!! Ridiculous!!
As far as being able to nurse.... it's supposed to be the most natural and easy thing in the world, right? (that was purely sarcasm!!!) I think what helped me with my first, was pumping a few ounces after she would finish eating.... I started doing that within the first couple of weeks and by the time she was a couple of months old, I was pumping 6-8 ounces from each side! I guess I tricked my body into making more??? Anyway, it might be something you could try if your supply seems to be struggling.
But the main key is just to RELAX and let your body do what it's made to do. And don't stress about it.... breast feeding is great if you can do it, but I know plenty of very healthy well adjusted kids who were never breast fed!