First you need to find out what flies and how you will be presenting them to the desired fish.
Then you choose a proper leader and a proper fly line. The fly line should mainly match in density (presenting the fly on the surface or deeper down) and in profile. It's important that you can handle the full head during false casting. Of course the head length should also match your typical avg. fishing situation.
Small water = shorter heads, large water = longer heads
Small wind = longer heads, strong wind = shorter heads
Little space to cast = shorter heads, lots of space = longer heads
Large flies = shorter heads (incl. abrupt front taper), small flies = longer heads (incl. softer front taper)
Furthermore it's important that the head has enough weight to make casting your flies as easy as possible. If you have high fishing pressure and spooky fish a pretty light fly line may be a need to have.
Make sure to ask your dealer if the fly line matches the temperature zone you are going to fish in.
In the end it's a mix of these recommendations and your fly line should match best possible. No doubt there is usually a compromise to be made in one way or another one.
Once you have the fly line and leader ready to present your flies best possible to the fish, you need a fly rod matching your fly line. The rod also should match (playing) the fish.
For example some fish may be caught on pretty small flies. This means you don't need much line weight to cast them. But if the fish are very strong you may want to choose a pretty stiff fly rod offering a serious back bone for the fight.
Of course the other way round might also be a proper way to fish. For example in Skagit casting we choose a heavy fly line in order to make casting heavy flies easy. At the same time we use a relatively soft fly rod matching the power of the fish in a proper way.
In general it's also a good hint to choose a more soft fly rod for casting below the rod tip and a little more stiff one for mainly overhead casting. It's also fair to summarize stiffer rods being helpful to cast heavier flies.
When fishing extra thin leaders for really spooky fish a significant soft rod helps to not break the leader when setting the hook.
Finally your reel should hold the fly line plus a serious amount of backing. There is always a large fish out there. ;) If you fish for very strong fish or in saltwater you may want a solid fly reel. Otherwise I personally would not spent much money for a fly reel that mostly will only be used to carry the fly line.
In short I recommend to go this way:
Fish --> choosing proper flies --> adjusting the leader and choosing a proper fly line --> choosing the fly rod --> choosing a reel.
Biggest mistake I see is in buying the fly rod first and then trying to find a fly line matching the rod and not the flies and the fish in the first place.
Oh, and pls. make sure you check with the company selling you the fly rod what exactly the fly rod was made for. You know one fly rod may be made for presenting dry flies on 80-100 feet distance while another fly rod may be made to present dry flies on an avg. distance of just 20-30 feet. Still both fly rods may be recommended for a 5 wt. fly line. Of course the first (distance based) rod would be significant stiffer.
Hopefully this will help some of you fine fly fishermen out there to find a proper tackle selection! A great hint is also to watch the tackle which the local experts are using. But be aware they may be on a different level of fly casting - thus being able to handle a different tackle selection not fitting best for yourself!
Great fishy week to all of you!
All my best
Some impressions from the last days. Proper fish involved as usual! ;)