I've found that people actually tend to perceive me as knowing more the more honest I am about what I do and don't know. I'm sure some people aren't that way, but I've found it to be true the vast majority of the time for me. I would consider fudging the truth the riskier option unless you're some kind of silver-tongued devil.

In any sane workplace, pretending like you know the answer to something you really don't will harm you far more than simply admitting you don't know, especially on the first day when you can't really be expected to know things.