Obviously, you should be comfortable diving & know your gear well before you spearfish. For some that may take years, for others weeks, but don’t spearfish until diving is natural.
For general purposes, a 42-48" gun is a good place to start. People who start with guns under 36" are often very disappointed. A 48" gun has reasonable range while being easy to handle.
Price & your budget will dictate brands/models open to you. Riffe (pronounced like "Life" with an R) makes GREAT guns for a reasonable price. A mid-handle Riffe will be my next gun. Lots of used guns are sold on ebay. Reality is almost any gun you find will work — to some degree. Differences are in penetration, range & accuracy.
5/16" spears are normally the best compromise for a starter gun. Thinner spears are faster but bend easier & don’t penetrate as well. Heavier spears kick a lot more & are slower.
Band guns are vastly more popular than pneumatic guns. Pneumatic designs are promising but band guns are reliable, cheap, and powerful. Someday a pneumatic gun will be made that uses a line from a person’s regulator to give it enough pressure to be impressive. Until then, stick with a band gun.
If you can, borrow or rent several different spearguns before you buy. If that’s not a reality, get something reasonably inexpensive but adequate for what you plan on hunting & use that until you are sure you will stay with this. If so, you will want to buy a good speargun, or 2 or 3.