What is the most weight i can lift

I might be starting a new job at a liquor store and some of those boxes can be heavyand i dont know how much weight is safe to lift. Im pretty healthy but didnt do any weight lifting before i was pregnant. I havent decided if im going to tell them im pregnant yet cuz i really need the job but if its unsafe for me then i know i have to right away. Please help im 20 wks.