Re: Fly Types.
Nymphs are like the caterpillar version of aquatic insects: its the crawling version that later will be a much more elegant flying creature. The difference is that nymphs spend all their time crawling around on the bottom underwater.
That doesn't mean you have to be scraping the bottom to catch fish on a nymph. Nymphs get dislodged from the bottom by current in moving water, and those in still-waters roam around a bit.
However, they can't swim very well in either situation, so you'll need to make the fly move either very slowly, or not at all relative to the current. They're most commonly fished with some weight (MUCH smaller shot than you'd use with a spinning rig) to get it to the bottom, and an indicator so you can see a strike.
Ideally, you'll position the indicator at a spot 1.5 times the water's depth away from the fly on your leader, and add enough weight so that the fly tics the bottom every five feet or so. If it looks more like a "take" than a "tic", set the hook. At first you'll feel silly for setting the hook at least ten times before catching a fish, but this is just how nymphing goes.