Wetdawg is offering good advice, and if you will use the search function, you'll see good advice from lots of good spearfishermen. Only to get you started before you buy any speargun, learning to hunt fish doesn't require equipment.
Before diving underwater, start your search with the local wildlife department for laws for spearfishing in your area. Find out seasons and fish you can take. Talk to other divers about what is good to eat from the list of fish you can shoot. Most won't tell you the exact spot, but will help you find the areas, and depths to hunt.
On every dive look for the small fish, usually the food for bigger fish. In our area, every diver sees small sunfish. . . then swims right on by. Small fish can tell you, what depth, and water conditions are good for that day. Small fish frequently are feeding on algae, so they are moving around like cattle in a pasture. During those travels they will move close to hiding spots for the predators. . . congratulations you've just found a "secret" hunting spot. If there is good cover, that spot will attract the predators for many years to come.