I am trying to put myself in your shoes of just starting out and trying to know what flies to use. It wasn't that long ago for me. Pardon the length of my post. You may find it helpful; I hope some of it is useful. Getting a good book from the library is a good idea too.
Here goes:
I think knowing basic things about trout food would help. Think of three basic food types:
1. baitfish
2. aquatic insects
3. terrestrial insects
Baitfish - always present/available; this is your minnow and there are many types. Streamers immitate these well. Think wooly bugger in black, brown green/olive and white. Once again, thousands of other immitations, but the wooly bugger is easy to find in stores, easy to tie and simple to fish. Any time of year, any time of day, trout will feed opportunistically on a baitfish
Aquatic Insects (the heart and soul of flyfishing for many of us). These are your nymphs, wet flies, soft hackles, emergers and dry flies. Nymphs are always available to trout. The pheasant tail and hare's ear pattern can imitate any nymph by varying size and color. There are numerous variations and pattern, leave that for down the road. Regardless of what is hatching and active, any nymph can catch a fish at any time.
The nymph is like a small crawling bug, but it lives underwater. It is the underwater stage of the mayfly and stonefly type of aquatic bug. When the adult insect mates, it drops eggs in the water, which shortly "hatch" into a nymph. (This isn't the "hatch" most speak of when talking fly fishing). The nymph lives among and under the streambed rocks, sticks and debris. If you have access to streams with good bug life, find a shallow riffle and look for flat rocks that are not imbedded in the bottom. If the rock is leaning upward with the high side downstream, lift it carefully and turn it over and look close-- with any luck, you will see a few nymphs. Nymph can also swim, but most crawl and the swimmer stay close to the bottom unless they are emerging (more on that later). Occasionally, while moving about, the nymph loses its grip and becomes "currentborne." Trout will search for nymphs but mostly, they lie in wait for these "currentborne" pieces of food.
The caddis has a slightly different life-cycle from the mayfly and stonefly, but there is a pupae stage where the caddis is a tiny worm-like creature. The trusty maggot, which in baitfishing circles attracts high esteem is about the size and shape of the caddis pupae. They will also become "currentborne" and fish feed on them like they do nymphs.
The adult stage of the mayfly, caddis and stonefly are immitated by your standard "dry flies." While a trout may take a dry flie immitation at any time, they are most likely to take it when the "emerging" or "emerged" adults are actually on the water. Many fly anglers will not even bother with a dry fly unless a "hatch" (actually the emergence of the nymph to the winged adult) is underway, or at least has been underway recently.
Terrestrials - land worms, ants, beetles, spiders, cricket, grasshoppers. When they are active on land, they often find their way into the water, mostly by accident. When they do, trout will gobble them up. They are mainly a topwater pattern, though some can be effective when "drowned." The main time for these is summer and fall, so I'll leave that topic now.
Enough science? I think so. What you really want to know is where and when to fish them. I will keep things simple. Trout like to sit in an area where they don't spend energy to stay put. Behind rocks, directly in front of rocks, in troughs or depressions in the stream bottom, in slow pools. They take up a position and this is called a "lie." Often you will hear talk of a "feeding lie." When most of the food is coming from underwater (nymphs, pupae, baitfish), they will be looking into the current for approaching food. They let the food come to them, and don't often move far to chase it.
When they are feeding on bugs on the surface, they will find a lie that lets them watch the surface in front of them for floating food (you will hear people say "the trout were looking up.") Quite often, when trout are actively feeding, you will actually spot them in their feeding lie and can see them move slightly to grab drifting food. When they are actively taking food of the surface, you will see them "rise" to the surface and sip, slash or attack the food.
Bringing it all together. Nymphs rarely reside in deep pools. However, when they are in the current, they are carried into pools, so you can fish nymphs in pools. However, nymphs are most effective in riffles, pocket water and faster runs or at the head of pools. Drift your nymphs around rocks and through and along current seems. Dry flies can effective in these same areas, but will also work in pools more so than will the nymphs. Baitfish can be fished anywhere as well.
That's enough to think about I am sure. Other posters may disagree with some of what I've said. Take it all in. I am a hacker, for the most part, but what I've told you here is a combination of things I've read and things I've experienced on the streams. Good luck in the sport. Getting one or two fish should be a great day for you at this stage, though I hope it will be much better very soon. In this part of the year, one or two fish is a great day for almost any of us, except the most expert or impatient.
