It doesn’t matter whether you are dinghy sailing or yacht sailing. The most important piece of equipment is your life jacket or buoyancy aid. Always make sure you have one onboard. Do not rely on others for your safety kit. Buy the best you can, it is your life it will save. My personal advice is to go and try some on. The more comfortable you are the more likely you are to wear it. From personal experience the Gill compressor vest is good for dinghy sailing while the Ocean safety Kru jacket and the Spinlock life vest are the comfiest life jackets I have worn.
Always carry a knife. It seems a small thing but if get caught in ropes or under a multihull trampoline then it can make a huge difference.
There are numerous safety devices out there pitched at everyone from the crew to the owner of the boat. Take some time to understand what is out there and then choose what is relevant to you. The RYA can supply you a list of items that you should carry on your own boat for different types of sailing.
The most important rule though is if it looks risky, it probably is so don’t do it…..