Going to your first kayak event is very similar to going on your first club trip except a higher level of endurance is required.
Firstly do you have the right kit and/or access to the right kit? If not get the very basics sorted. It’s going to be a miserable event if you get really cold. Make sure you have a dry set of kit to get changed into at the end a spell on the water, something nice and warm is always a great idea and include a hat.
A large beach towel is good bit of kit to get hold of. You’ll need to put this gear into something and also be able to carry your wet gear home with you without getting everything else wet. Ikea blue bags and plastic “bags for life” are superb for this job and despite not being billed as waterproof they hold up very well.
Most events take place over a few days so keeping warm on/off the water can become an issue. Multiple changes of warm kit will be useful. The chances of being able to dry wet kit off will be pretty slim unless you are lucky with the weather. A couple of different bags to keep dry gear away from wet gear would be very useful. Something like a dry bag for your dry kit and the Ikea blue bag for your wet gear works well.
Many events involve some sort of overnight accommodation such as a tent or bunk house.