Careful, I could bore for West Sussex on this subject.
1. A decent gun over 60cm no more than 75cm. Mine is a Beuchat Mundial, Cressi Sub and Sporasub are virtually indentical.
2. Obviously, mask,wetsuit,snorkel and fins.
3. Weight belt, i use 3 yellow dive weights. This gives me equal bouyancy to offset the bouyancy in me and my suit. It makes it easier to dive and stay down. You must leave enough breath on each dive to swim to the surface. It sounds obvious, but when diving without weights you'll surface without trying and it requires no effort, with weights it takes a bit more effort. Hovering a couple of feet off the bottom is great.
4. A keep net. Returning to the boat/shore with every catch is a drag.
5. Surface marker bouy. Unless snorkling tight to the rocks/shore it makes sense, I tie the keep net to it also.
6.Knife. With the string from the marker bouy and the recovery line on the gun around you, if you get snagged, you wanna be out of there pretty quick.
That's about it, find a good spot, about 15ft of water, where the sand meets the rock/weed is good as you can see the fish moving in and out of the weed. In thick kelp it's hard to spot them If the water is too deep and the vis is not great, you'll have to constantly dive to spot fish ,which rarely leaves enough air to line up a shot.
The rest is practise.