Excellent advice offered above, initially just go in with your mask, snorkel, suit and fins to get a feel for them, swim against any current, get a feel for the conditions and what you can and cant do, as Woohoo said stay close in, get comfortable being out of your depth, get used to treading water with your fins, taking your mask off and having your hands free etc. You cant practice with your fins enough, they are your propulsion, you need to know what they and your legs can do, getting into strong current with fins or legs that cant cut it is frightening, research the conditions that you will be diving in, initially stick with the conditions that you know. Then introduce the belt and weights and the first time you are just working out how much you need, the first time I went out I had to hide my excess weights under some shingle as I took too much!

Don't just put the weight on the belt and go straight out into the deep, to stop and then realise you are overweighted, find that out very, very close to shore!!

Research the subject, shallow water blackout etc and know what can go wrong and why it can go wrong.................. knowledge is power!!
When you are comfortable with your mask, fins, snorkel and weighting, introduce the speargun. I forgot to say, whenever you enter the water do not forget your knife, practice taking it from its sheath, don't wait till you need it, know how to do it before then
When your comfortable being in the water with your gear then add the kayak and remember that you will be carrying more weight so depending on the volume of your kayak, its handling could be very different. This could make a big difference on long trips, my kayak handles very differently with my diving gear on, to be honest I need a bigger kayak for diving but everything costs........
Good luck, trust your instincts and have fun, nothing compares to being in the sea, its a tonic and its free. Be careful though, its highly addictive
