Make sure that you know exactly what seats exist in the theatre. Get a seating chart, and compare it row by row to the actual seating area, to ensure that it is 100% accurate. If there are any surprises, you want to know about them before the audience arrives. It makes a very bad first impression if patrons purchased tickets that don't actually exist, or if you promised somebody good seats, but you actually put them somewhere with an obstructed view. The list of possible issues is endless, so you need to be sure you are totally familiar with the space.
Make sure you have access to the complete list of tickets purchased to any and all dates of the production
Make sure you have the will-call tickets, and that they are organized and ready to hand out
Make sure you have the final list of all tickets purchased online (online sales for each day typically end 60 minutes prior to the show)
Make sure you know exactly which seats are available to sell to patrons who don't have tickets yet
If you will need change (cash), make sure you have it (and that you know who is providing it, be it you or somebody else)
Greet patrons with a smile as they arrive at the box office ("Hello, welcome to ________, how may I help you?")
Make sure you mark off seats as you sell them, so that you or somebody else doesn't accidentally sell the same seat twice