Just a few tips in addition to what's been said already.
1) Avoid sitting next to the boat engines - I find it can help me be sea sick.
2) Drink plenty of water before the dive, on the boat and after the dive.
3) Avoid greasy food - eat cereals and non-acidic fruit.
4) Talk to your buddy - if you need help kitting up don't be afraid to ask - look on it as your pre-dive check and always do your pre-dive check no matter who you are diving with. Try to kit up as soon as you can without rushing - it'll make you more relaxed for your descent and you won't be sweating as much.
5) If you feel uneasy about a dive don't do it - you can always call a dive for any reason. If your buddy gives you grief about it then find a better buddy. Good divers appreciate honesty before a dive - they don't like nasty surprises in the water so if you have any misgivings before you enter the water talk to your DM or buddy first.
6) Plan your dive, dive your plan - talk to your buddy about who is going to do what - make it part of your plan. Review your tables before you dive even if you have a computer and go thru' hand signals. (The DM is likely to do this too but you may need to talk to your buddy too)
7) Listen to the skipper and boat crew - when you surface make sure you establish positive buoyancy before getting back on a ladder/boat and don't rush unless sea conditions dictate otherwise.
8) Take your time, monitor your air and watch your buddy at all times but enjoy the dive too.
9) Come back to Scubaboard and make us all jealous
10) Have a great holiday !