Play a sorcerer. It's easy and your ignorance will work well with the role playing aspect. I'm not talking down to you. Everyone is new at stuff in the beginning. If required have them pick your spells. There are so many rules to work with at the beginning that you will want some help. Magic missle and mage armor are easy while you watch and learn the rules from others during play.
Read the material. It'll help you get the hang of things.
Pick up a Dragonlance novel to start culturing yourself.
Don't be afraid of death. Don't run to it, but don't be to angry/sad if it happens. There is no challenge without risk of failure.
Take food and/or drink. It's a gathering. All gatherings are better with food.
Remember, the DM owns the game. Yes there are rules, but these rules mean different things to some. To some these rules will be a loose guide, and to others these rules will be an instruction set written in stone. Some rules will conflict with others and/or physics.
Physics have no bearing on magic.
You don't have to know everything your character does, and the character will not know some things that you do. If in doubt, ask the DM. They'll probably put it to a die roll.
Be careful. Dice are addictive. Scientists haven't discovered why. If you do buy dice, try to get a unique set that everyone can point out as yours, or go with the grab bag (pound o dice) that you don't care about. I have two sets of ~60 dice. One matching blue and one matching red. They're easy to identify which is good as we sometimes use them as character markers on maps (marching order, attack formations, etc)
Throw your dice out where they can be seen by all. If you get on a winning streak, no one will doubt. Kill anyone that touches your dice. It's the only way to get the taint off.
Learn what temperament (alignment) you like to play. It can be hard to play a neutral or evil if you feel bad about doing indifferent or evil things. Playing a lawful character can be hell to a chaotic personality. I'm rather goofy, and sometimes will do the right thing (other times I will do what I think is funny) so I usually enjoy playing chaotic neutral.
Don't take events in the game personally. It's a game. The Neutral evil rogue character might steal from the party. It doesn't make the player a bastard. It doesn't make you a bastard when your character kills his character. At the same time if your sorcerer accidently hits the party with a fire ball because it (and maybe you) are ignorant of it's power, they should understand. Of course, the cleric may get irritated if you keep doing it. You may not like what others are doing... they may not like what you do. Don't take anything personally.
Don't be afraid to stick with a class. Don't be afraid to experiment with multi or prestige. In my experience, I've found the base castor classes best when they stick with the class. Melee can be better in prestige.
Don't be the guy who is out to break the DM's world. They die the fastest.
Good luck. Have fun.
