Welcome to the forum.
If you have zero experience, then there's a lot you can do on dry land so you know your way around the boat and engine. A standard issue recycling bin is perfect for getting the engine mounted, and run in so you get used to checking the tell-tale cooling stream, choke, fuelling, etc. Killcord will be second nature too. Tube pressure should be rechecked on the water.
Clothes, then don't skimp on decent kit. Waterproof separates including trousers with adjustable braces and jackets keep you warm. Invariably there's always wind and showers in UK waters even during summer months. XM, Musto, Helly Hansen, take your pick. Pair them with wellies and waterproof gloves. Don't forget sun cream. There's plenty times I've come back to shore like Peter O'Toole like Lawrence of Arabia!
Life jackets for everyone on board. I like auto-jackets with crotch straps. Again, plenty reputable makes. I like Seago around 175 Newton capacity. Remember and check every trip. You can service yourself in 2-3 years with recharge kits.
Safety gear, then sit your VHF course and get used to marine traffic. It's a reassuring piece of kit to have on board. Small anchor, rope and chain, waterproof torch, diver's knife in a scabbard, spare spark plugs, socket to remove and dry rag, spare kill cord and fuel.
Dry bags are useful for everything you want to carry including your lunch. Don't spend a fortune, they last a few seasons before they spring a leak. Lidl have them on offer ever so often.
Lastly, ask locals in the harbour you're launching from. Most folk are decent and it's good to have a natter before you leave.