I would recommend first of all, once you get certified, don't spear and just get experience in the water. You need to be adept with your trim( bouyancy) and comfortable in the water before you start slaying stuff.
I am working with a friend who is doing just that right now. He is not ready yet after 15ish dives.
When you do get certified, don't buy anything related to dive computers or bcd(your harness that holds the tank). The type of gun you use will come into play here. If you are reaching in a hole to grab a fish, anything on your wrist can get in the way (I use a wrist mount computer fwiw)
You will need to figure out what type of gun you prefer....hip or chest load, or foot (pneumatic)
Chest is designed for freedivers. Hip loading guns (with longer but stock) are designed for SCUBA. Of course both can be used in either application. Pneumatics, well, can be a pita to load (for me anyway), but are poweful and sound like a .38 spl going off.
There is a ton of other options, reel, open or closed muzzle, free shafting etc.
I would recoomend not getting anything shorter than 48". This is another
as how guns are measured vary.
Sharks? If I drop and see a bull or two that stick around, I won't shoot.
PSD's that slip on the end of your shaft are an option. I don't use them, but poking a shark with a loaded shaft works, sometimes. I have been pushed several yards by a shark between my legs while I pressed the shaft to his head. Shaft was in place, but not loaded.
It's a great sport. I am selective though, shooting things I like turning into tacos.
Oh, IDing and measuring them underwater is a challenge.