This is a good question! If you’re a man, then that should be fairly obvious!
However, when it comes to the ladies, it’s not so straightforward. If you like to sing high and can make a half-decent sound when you do, the please head for the Sop 1s. Slightly lower, then the sop 2s and if you sing in your boots, the head for the altos!
Having said all that, someone will direct you and help you on your first night, so don’t worry!