If you are expected to bring a character you should definitely bring that.  If you're going to make a character there you should come with an *idea* about the following things (but remain flexible, you idea may not be exactly what you end up with).

 - Character name
 - Character race (Human, Dwarf, Elf, *Halfling*)
 - Character archetype (loyal knight, scheming wizard, back-stabbing rogue, etc)
 
As far as rules knowledge, the Players Handbook (PHB) helps but it will speed things along greatly if you know these few simple rules.

 - For most things you will attempt (including attacks) you roll a d20 (that is a twenty sided die), add a modifier, and compare it to a target number.  If your total equals or exceeds the target you succeed (hit, pick the lock, whatever).
 - You have abilities.  Some you can use all the time, these are called `At Will`.  Some you can use only at intervals. These will be `Encounter` or `Daily`.
 - You have hit points and healing surges.  These represent how alive/tough you are.  If you run out of hit points you start dying.  You can use healing surges to regain your hit points *in certain situations*.  A healing surge *usually* regains one quarter (1/4) of your total hitpoints, rounded down.
 - We refer to dice by the number of sides they have, preceded with a *d*.  A d4 means a four sided die.  3d6 means three six sided dice.
 - When in doubt round down.
 - The DM overrules the rules.

Most of all, remember you're there to have fun!  The other players and DM should be willing and able to help you with the details