Yup, takao's right on. I'm not a pro, but I do enjoy spearfishing. I like 3-prong graphite spears (hawaiian sling in mainland speak

)in the 8' range for day diving, you need the speed. If you can get in close with a three-prong and get fish, you can definitely get em with a spear gun. For night diving, I like the heavier spears, fiberglass, and even aluminum sometimes. You can often find bigger fish (parrot fish, for example) simply sleeping, and the heavier spears are naturally better for the bigger fish. My uncle taught me how to spot octopus lairs in the reef; anything with a tip will work for that as you don't typically spear them.
Like takao said, practice with a buddy. Know your limits, don't push yourself too hard. Sometimes one gets tunnel vision on the hunt, and you forget to breathe and consequently black out. I don't need to tell you how dangerous that is. To work on holding your breath, start with holding your breath a few times during commercial breaks while you're watching TV; it gives you a regular training period and no one looks at you funny while you do it
Also be careful when you pressurize your ears, there is a risk of barotrauma. There are various methods to properly achieve equilibrium underwater, i can send you a link later if you want Jake. At work now, but I'll be back later.
ETA: looking for the link now, Jake.