For fishing purposes, I second Boot: learn a few flies and how to use them and you'll be off to a good start. I usually lug around several hundred flies (been tying for 20 years and I'm bad at cleaning out the boxes) but I usually only fish about 6 of them and only catch fish on about three of those.
If you're interested in learning to identify flies for academic reasons, which can be fun, try Fly Anglers Online at
flyanglersonline.com (I just Google FAOL). There's more fly fishing information there than you can shake a stick at (and you can shake a stick at quite a bit of information!).
This thread on whatever forum Google took me to seems to have some good resources too.
Happy Fishing!