That's quite a challenge for Day 1, but don't worry yourself too much.
As Andy says, decide if you are telling people what you are doing - as it is a work colleague thing I'm guessing you might not be wanting to tell them, at least not right now.
In that case, I'd do as BL says, say you're not feeling too well and have taken some medicine or somethin, but won't be eating/drinking as you don't feel up to it.
Social gatherings are the real big challenge when abstaining, especially ones based around food.
But I know you can do it, and just remember, whatever anyone says of thinks about you being on LL, you are doing this for YOU and not them.
Hope you have an enjoyable time
