Matching the correct weight of fly line to the rod is critical for the rod blank to work to its full potential when making a cast. If the line is too light or too heavy for the rod you will without doubt struggle to cast.
To make sure you match the line correctly just look at the rod blank near the handle and you will find some numbers after the rod length telling you which weight of line it was manufactured to cast. These numbers are referred to as the AFTM scale (Association of Fishing Tackle Manufacturers). The number on the rod may simply say 7 weight or may read as #7 or even 6/7 giving you two line weight options. For the beginner I would always advise opting for the heavier line weight in this instance as you will be able to load the rod easier and have a greater feel for the cast through the rod blank.
Fly lines have what is known as a profile, the two most common being Weight Forward and Double Taper, both are abbreviated in fly fishing world to WF or DT.
The WF lines are most common these days purely because they are the easiest to cast and achieve distance with. These start with a thin running line on the reel end, then progress into a thicker heavier section of around ten yards in length towards the end of the line before tapering down thinner again at the very tip of the line to give good presentation. Because most of the lines weight is at the front this enables easy “loading of the rod” (bending the rod) and gives good distance when shooting a line.
DT lines are the same thickness throughout their length and taper down in size on the last couple of feet at either end. In the right hands these lines offer better presentation of the line onto the water and are better suited to river fishing where you may encounter wild fish that are “spooky”. These lines have the added bonus that if they wear out on one end you can simply turn them round on the reel and use the other end as the profile is exactly the same.
Finally the “density” of the line is how it sits in the water and this can vary from floating on the surface to a fast sinking line with a sink rate of 8 inches per second to get your flies down deep fast. As a beginner always start with a floating line as these are easier to cast than the sinking lines, then progress to sinking lines as required as your casting improves and to be honest most of my own fishing is done with a floating line anyway as usually the fish are in the top layers searching for food, I only usually turn to the sinking lines in extreme conditions of hot bright sunshine or very cold conditions when the fish may drop deeper in the water.