Get Lee Jones' book - Low Limit Texas Holdem. That is by far the best book I know of for absolute beginners on the game. Read a little bit first just to understand how the flow of the game goes. 2nd, try downloading one of the major poker rooms software if you can. They have play money games on there that will at least get you used to the flow of the game. You wont find necessarily people playing strong games on those tables, but it will get you used to what the blinds are and how they work, how the betting rounds go, when to fold, what a checkraise is - things like that. It wont necessarily be a barometer of how well you will play, but it will at least get your feet wet in seeing how a live table operates.
At an office tourney like that, just have fun. Chances are you will have at least 2-3 Rounders wannabees there, who will scream and holler all night "What you called a $4 raise with XX when I had XX and you were only a 45% chance to win with your 4 outs?" I've seen azzholes like that, and nothing puts a buzzkill on a fun night more than a couple of cardjockeys like that. So if anything, have fun, and be fun to be around - especially if it is your first time so you will be invited back. If you take a bad beat, tap the table and say "Nice hand sir, well played"