It is worth looking for a second hand kayak on Gumtree/ebay etc as you can quite often get something set up with fish finder, paddle, seat etc for less than a new model. Length of the kayak will be dependent on storage space and where you intend to fish. If you want to go onto larger waterway and open ocean then something 4meters or more is recommended. For rivers etc the under 4meters will be fine. Also check the kayak can float you and all your gear with plenty of capacity to spare.
Possibly the most essential skill is being able to remount the kayak in deep water. If you can't do this you could end up in serious trouble faster than you realise. Take the kayak out a couple of times and get use to it before you start taking fishing gear. Jump off in water you can't stand in and try getting back on board. This will also help when mounting accessories as you will know which areas you need to keep clear.
Once you are ready to start fishing, keep it simple to start and leash everything. Rods, pliers, nets, etc etc. I think everyone I know who fishes from a kayak (including me) has lost stuff because they thought something would be fine. There are plenty of instructional pages on the internet for making your own, just pick something that suits you.
Try to get a kayak with internal storage and a decent dry bag for your wallet, phone, keys etc.