To try to get you off to a quick start, you must realize life underwater is between hunters and prey. Not only divers are the hunters. All fish are hunted, every day of their lives. Some of the fish are predators, other fish will eat scraps of plants. What this means as a new spear fisherman you have to approach fish in a neutral manner. Not threatening. From all of their lives, a predator approaches close, head on then lunges forward to grab a meal.
As a spear fisherman, move at an angle to the fish you want to shoot, don't stare at your target, Look sideways ocassionally. Make your approach patiently, anticipate where your prey is moving to, and get there first, and let your prey approach your side. . . our spearguns work sideways.
Before you get in the water, talk to the local fishermen. You want to know what fish to hunt, and what their behavior is like, how deep do they stay, do they move around to find food, or do they hide at an ambush spot and wait for the food to approach their hiding places.
If you are new to diving, your underwater behavior will scare fish beyond your spearing range. . . dive a lot and your diving skills will let you swim through the middle of a group of fish. . . watch the videos, sharks can swim next to fish, if they don't appear to be hunting. . . when their behavior becomes aggressive, fish flee from them.