Picking a good one

First of all, don't choose the soldier palmer. I hate it and I'm convinced that its enduring popularity is part of some weird conspiracy!
I suppose the next thing to consider for the newcomer is that the flies are working together as a team, sometimes a fly might not actually be getting eaten, but is drawing fish in to have them another fly on the cast. We don't always realise this until we take it off and all the flies stop catching. So when I think of my team I generally think of it as tapering down from top dropper to point. So the top dropper should be the biggest, most heavily dressed, water pushing pattern, the middle dropper should be a bit smaller and sparser, the point being slimmer still. A nice early season cast for the lochs where I grew up would be bibio, palmered coch, blae & black. It's not always like this but it's a good starting point. You might for example want to fish 2 larger bright attractor patterns; one on the top and one on the point with a smaller more natural pattern on the middle to give the fish a more edible option and put a bit of istance between the gaudy offerings.
Or on a summer night, you could fish two smaller sedge patterns on the droppers with a muddler on the point to hold the team up near the surface.

Then you have to decide which colours you are going for. The old addage of bright day, bright fly is reasonably reliable here, but can definitely be refined. Bright and dull isn't quite enough in my opinion I like to think in terms of harsh, pleasant and low light. Then there's water colour, peat stained, clear or with some degree of trubidity all has an impact.
In harsh light I like White, orange, silver blue and black, but not claret, olives or fiery browns. Sometimes reds can be good, gold if loch is peaty-I love a doobry on a peaty loch on a bright day and it's a definite first choice for top dropper, but that's pretty much the only situation I like it. In pleasant or good light, those days with a nice bit of cloud cover and plenty of difuseed light around go for hare's lug, yellows, reds golds, olives, greens and browns. Avoid blue & pink, but other colours like black, claret or peach can be useful.
In low light claret and black are essentials. White, silver, pink and peach are also worth trying. Yellows, orange and gold flies can stay in the box.

The dirtier or more stained the water, the more intense the colours of your flies should be. Fly activity also affects this, if there are fish eating olives on a bright early season day with a good wave then suddenly I'll like olive on a bright day!

Then we come to size and heaviness of dressing. The bigger the wave the bushier the flies as they move more water and are easier for the fish to locate. The calmer the day, the lighter the flies should be dressed. Muddlers are something of an exception; they're useful from near flat calm to a rolling wave, but still match the size to the wave. Looking back at the teams I mentioned earlier, assuming the same light conditions but different winds. On a big wave I might have a size 10 heavily palmered bibio muddler, a size 10 palmered coch and a size 12 blae & black, but if it's a warmer day with light winds and just a bit of a wave it might be a size 12 bibio, a size 12 palmered coch and a size 12 or 14 blae and black. Just a wee ripple, size 12 bibio snatcher, size 14 slimline coch zulu, with a 14 blae and black -possibly lighlty muddled- on the point

I'm sure there will be plenty of people who disagree with this, or at least some parts of it, but it's what I've found to work for me over the years. There are exceptions, and sometimes I'll choose a fly that seems wrong for the day but it works, there's probably some observation that's being made subconciously. But that's down to experience, I'm sure we've all experienced those times when we decide to do something but can't quite articulate the reasons for it, and yet it works.