I know by asking this I'm opening a huge can of worms, but when you first walk up to a trout stream, how and why do you decide what to tie on? And when/why do you decide to switch it up? Do you always tie on 'old faithful' to start and the go from there based on what you observe, or do you look for other clues, ect around the stream? I've never had much confidence in what I tie on the end of my leader and would love some insight into what you do when you hit that first hole!
If I'm fishing new water, then probably the first thing I do is look at a hatch chart for the particular water to get some idea what to expect for that time of year. I also like to look at what the local fly shops are saying in the fishing report. For new water, I will stop at the local fly shop, ask some questions and ask them to help you pick out a few flies.....I always buy a few flies from them to help compensate for their help. Next when I get to the water, take a hard look around, are fish rising, are there bugs in the air or on the water.....sometimes they are hard to see, get down close to the water and take a very hard look. You might try turning over some rocks to see whats crawling around. Then take a look at the nearby bushes to see if there are any bugs on the leafs. One of the things I like to do is sit down on the ground when rigging up, you will be surprised at what you will see crawling around......this also gives you some time to survey what is happening on the water. What time of year is it, is it hopper time, if so I like to use a hopper/dropper rig. Don't forget about ants and beetles, trout love them. You might also try walking through nearby grass to see if there are any bugs moving about....if you see hoppers that will give you a clue on the size and color.
It all depends, sometimes I start with a Old Faithful or if I know there will be a hatch then I match it, best I can. Sometimes I just feel like swinging wets or streamers and will do that till I see a reason to go dry.
Unless I see something hatching or know what to tie on from a hatch chart or fly shop advice, I tie on a 14 or 16 parachute adams because I love dries and love the parachute adams. Doesn't always get a strike but I'm surprised how often it does.
First thing I do when trout fishing, is look for cedar winged wax bills to see if a rise is on. Then, I watch to water for rises. I either watch to see what bug is coming off or walk the shore looking for what bugs are trapped in spider webbing. If all of the above fail, I throw a size 16 parachute adams
I almost always start with dry-dropper setup (unless it is winter). From april-october you would be surprised how well a small hopper and copper john will do in a variety of waters, at least to start the day. And then adjust based on this:
Warm <------------> Cold
Turbulent <------> slow
Big water <-------> tiny creek
Warm weather <----> cold weather
Hatches <------> no hatch
Big flies/streamers/nymphs <-----> tiny flies/midges
Warmer/turbulent/larger waters I throw bigger brighter stuff
Cold/small/slow waters I throw smaller more natural stuff.
Of course tailwaters could be cold/big/turbulent or slow, but generally use small midges. You will get a feel for size and type of bugs based on season and the above water characteristics. But generally colder= smaller bugs. Try to match color based on the naturals or the substrate color. Even when you are throwing #22 midges, sometimes a bigger attractor works to draw the fish's attention to the midge. Think mechanistically; sometimes the fish would prefer desert over dinner. If 1000's of naturals are coming off in a hatch, even a really good fly is just one of 1000's in a drift. Try something modest but larger. Remember, fish are not entomologists, patterns that look like many things can do very well (PMX, copper john, birds nest, various soft hackles, hares ear, prince, adams, elk caddis, all just look 'buggy' and can imitate many different bugs).
I have about 6 go to flies that I'll try first. The Brown Hackle (and variations of it), the Pink Mystery, The green machine, white miller, black knat, butterfly. Then I'll get pickey and chosey and check what is around for hatches and time of the year critters.
I usually tie on a number 12 olive woolly bugger. It almost always catches fish which means I really suck at nymphing because I hardly ever do it. I really need to practice more but I just cant force myself to nymph fish.