Be early, be cheerful & be relatively quiet (when nervous it's easy to chatter).
Run though setting up your gear, getting dressing, donning gear, the whole dive and boat re-entry and undressing/stowing stuff all in your mind so everything will be a little more familiar & fresh in your memeory when it's the real thing. Mental preparation is great for calming one's nerves. Make the first mental run-though at least 24 hrs in advance.
Seasickness is greatly helped by starting the medicine the night before, also don't eat overly much or exotically that morning.
Make yourself a WRITTEN list and check it twice. Also re-check it when you think you took everything from the house to the car and from the car to the boat!
Before you are even ready for a buddy check, check yourself: breath each reg 3x while watching pressure gauge (mouthpieces intact, tightly attached & feel right?), check that BCD inflates & deflates, if you have a computer/light/other gear check that it works. Lots of stuff can be checked too but the 1st 2 items are the biggies: if you KNOW you can float & breath most other problems are fixable.
Better take something to eat & drink, otherwise Murphy (of the Law) will make it turn out that such was not available on the boat for whatever reasons. If nothing else maybe you can offer some food & drink to someone who forgot their wallet, or have a snack for the drive home.
That's all I can think of, except that if you haven't been out on boats much, do make sure you know how wet or how chilly it might be out on the water; I've no idea where Tob-whateveritwas is. A light watch/skully cap that covers your ears is always a good idea to have in your bag.
Still having no idea where Tober* is, if you are diving the sea or other big water it would be a great idea to take a SMB & a whistle. Let's just go with the idea that if you have them you will never need them so you never venture into the vice-versa.
Hope you have a wonderful time!