How to be an infield umpire?

I'm filling in for my friend who umps baseball, and I have to ump in the field. Now i know baseball and what I have to do, it's just where i should be standing at every moment is what worries me. So when no ones on base, should I stand behind the first baseman to watch for a play at first? Then if there is a guy on base, do I stand over by second base?