On the IC side, pick a nation but don’t feel bound by your initial choices. PD are very happy for new players to completely change their character if you find something isn’t quite as fun as you thought it would be or realise you missed something awesome.
Having somewhere IC to sit down and have a drink and a bit of shade is great. If you don’t join a group with a tent then find out where the social gathering place is, retry much every nation has them.
Plan your costume choices assuming mud and rain. Wellies and thermal under layers are not inappropriate.
On a more serious note, the ground can become very muddy, although PD will do everything they can to minimise it. Boots with zips will become clogged (pack a stiff toothbrush for cleaning them), and a pair of slip on OC Wellies are a good idea for things like going to the toilets and showers when you’re in the OC area. If you have normal base layers under your clothing (a good idea if you can) then bring a couple of changes.
If you’re coming by car and have space available a raised camp bed is one of the best purchases you can make, if not make sure you have plenty of insulation under your bed as well as covers on top. Don’t get a cheap tent, the extra money is definately worth it, and try for one you can stand up in if possible. A 3 berth tent is the minimum size you should be considering, each berth is literally a space for a person in a sleeping bag, so normal camping gear takes a second berth, and your IC gear takes a 3rd.
The event has catering, with a good mix of caterers on the OC field and a few IC as well, although you can also bring a camping stove and cook your own food. There is 1 bar which serves drinks for real money, and many many bars which serve drinks for IC money. You get a small amount of IC money (18 rings) which is typically enough for several drinks, and have a choice of resources which you can use to either do things in game or get more money (enough to get really really drunk on in the IC bars)
Other things to consider - if you’re camping solo, make sure you don’t need help getting into or out of your costume. If wearing a dress or robes go for a raised hemline or make it possible to raise your hemline at need (I wasn’t kidding about the mud). Have a warm cloak or over layer, even it’s hot during the day it can be cold at night. Put on sun cream even if the day is overcast, it’s much to easy to forget, and keep a big bottle of water in your tent, it’s easy to forget to drink while IC.
If you want to fight then you’ll need a monster costume, this can be plain dark clothes. There are 2 big battles each event, and if you fight 1 as your character you’re expected to fight the other as a monster (which is really really fun). You don’t have to do combat if you don’t want to, Anvil is safe and there are plenty of non-coms who stay behind during the battles.