As a fairly new player to D&D myself, here's my take:

The most important thing to have full knowledge on, during a game, is your character.  Know your character sheet inside and out.  Aside from the sheet itself, other required reading material for this would be whatever Player Handbooks, Dragon articles, etc. contain your character's class, race, feats, powers, and magic items.  Knowing your character - especially the feats, powers, and magic items - will greatly help the gameplay to run along smoothly.

The next, yet almost equally-most important thing to learn is the basic game rules.  You need to know how skill checks are handled, how things like line of sight and line of effect are determined, etc.  For this, the one definite must-read item is the Essentials Rules Compendium.

Study and knowledge of the above may still leave some important questions to be asked.  For this, you can check the D&DI Compendium (if you're a D&DI subscriber), ask Wizards directly (they have AWESOME support, in my experience) via phone or their website, consult Google and other online communities, or (as they will be the final word anyway) ask your DM before the session starts.