(Not sure what the protocol for this is: I just replied on the stackoverflow post. Copying and pasting here for now, and I'll delete one if anybody objects)
The fact that you're nervous is good :) If you were going in expecting to be writing a brand new, full optimized multithreaded physics system, you'd be in for a lot of disappointment (and they'd be in for a buggy system).
When you first get there, it might take a day or two to get your compute set up, get all the software installed, check out all the game code and assets from source control, and build and run on your target platform.
Your boss likely has a plan to get you up to speed on an existing component of the game; you'll probably start out with fixing a few existing bugs from the bug database to get the feel for the engine and the workflow. Someone (either your boss, or more likely another programmer on the team) should be keeping an eye on you, reading over your code changes before they get checked in, answering questions that you have, and giving hints on where to look in the code. As you get more experienced with the system, you might start doing more advanced stuff like adding a new feature to part of the engine.
As for my personal experience starting in games: when I started, I inherited the build system from another programmer that was sick of it - making sure that the code compiled every night on all platforms (Xbox, PS2, and Gamecube at the time) and could load all game levels (nowadays this is probably automated with e.g. CruiseControl). I eventually moved from that onto the menu system for the game, working with an artist to hook the game logic with the UI elements. I eventually left the company after burnout from E3 crunch time and joined the middleware company where I still work today.
One last thing: you said in another comment that you "don't want to be the reason they fail". That's impossible. If by some chance the game failed because of something you were working on, it will be your boss's fault for assigning you to something so critical when you're so inexperienced, and for not monitoring you on it.