From the reel to the hook
Fly fishing line isn't like normal fishing line. You have backing, line, leader, tippet. All of them have a purpose and a place.
Backing is the "line" from the reel to the actual line. How much you need depends on a few things like, reel size, arbor size. But there is an easy way to get it right. Back to that in a moment.
Most of what's on your reel is backing. It's bright colored and heavy. Easily seen by us. It's also there in case you hook a big one and he runs past your line length. I hope for this on every cast. Experience has taught me not to expect it though.
The line is next. It's tapered from one end to another. Usually the heavy, or thickest end goes toward the fly. It comes in a variety of weights, like everything else. The one you need depends on what you're fishing for. Like the rod and reels, 5 is good for most things.
Line also has density. This is what determines if it floats or sinks. Once again it depends on what kind of fly your using. Dry flies = low density. Nymphs = mid. Streamers = high density.
Pick a color "YOU" can see clearly. Try and think ahead. Will I be able to see this if it gets cloudy? How will it look in the evening? It's not going to be seen by the fish. That much because of the leader and tippet.
The leader goes from the line to the tippet. It tapers from one end that matches the fly line. It's clear so the fish can't see it. It also keeps the line from slapping the water and spooking the fish. It's a lot lighter than the line and backing. It "cushions" the landing for the fly.
The tippet attaches the line to the leader to the fly. It's translucent too. All you want the fish to see is the fly itself.
You want the strongest tippet you can have but still be unseen. And also, light. No extra slapping or ripples allowed down to the fly.