Before you learn how to cast a fly rod, you need to learn how to hold it.
There are three types of grips fly fishers use: point, thumb and golf grip.
The best tip I could give about choosing a grip, is to try them all and go with what feels most comfortable for the water you fish most often. Once you get the hang of all three, you can pick and choose which one works best for the specific fisheries and conditions you encounter.
A closer look that the three types and who should use them:
Point Grip
The point grip is usually the easiest to learn and is the one I’d recommend for beginning anglers. With the rod pointing straight ahead and parallel to the ground, start by pointing your index finger and placing it over the top of the rod handle/grip. New anglers seem to prefer this grip because they can hold the line with their middle finger, and get a better feel for each cast. With this grip, the rod is almost an extension of the index finger and the angler can point to the direction the rod should swing on the forward and back cast. This is a good grip for finesse casting on smaller waters.
Thumb Grip
Another popular grip is the thumb grip, which places the thumb (rather than the index finger) over the top of the rod handle. In this grip, the line can be held with the index finger (instead of the middle finger). This grip is used by more experience anglers who use a bigger rod, cover more water and get added distance on casts. This is a good grip for fly fishing on lakes or flats where long casts are necessary. Be sure to try to keep your wrist as straight and stiff as possible with this grip. Bending the wrists repeatedly on power casts can lead to pain and or injury.
Golf Grip
Similar to the thumb grip, this is the easiest grip to teach an old golfer as the thumb and forefinger are wrapped around the side of the club, I mean rod, as if holding an iron. This is probably the most comfortable grip, but, like the thumb grip, it can add stress on the wrist. The line can be held under the index finger in this grip as well. This can be a useful grip when trying to battle a breeze or to fling a large fly over a good-sized river.

