Pre-show you should monitor sign-ins for chorus/supers/dancers/children, make sure artists are on time (not only for their arrival, but to wigs & make-up). Check all prop & costume presets. Typically, during the show you'll be cuing entrances on one or both sides of stage, making sure that the performers walk onstage with their correct props/costumes and being the SM's eyes and ears backstage. You may also be asked to escort the maestro from the pit to the stage for curtain calls.
Going into tech, you should have a working knowledge of your set so that you can troubleshoot problems ("um... Isn't there supposed to be masking flown in here?"). Very often the SM is in the house and not backstage during the first few techs, so they will be dependent on you to communicate what is happening that they cannot see. The 30 seconds that it takes to get the chorus lined up into their entrance positions can seem like 5 minutes if the director and SM don't know what is causing the delay. Most stage managers would rather have too much information than risk not getting the one critical bit that saves the day!
Good luck, and feel free to PM me with more questions.
