On the First page of the Rotator there is usually noted a Duty Roster - which rotates weekly.
There are at the moment 3 possible positions you will be asked to perform.
1.  Welcoming.  This is an important duty that allows you to meet all the members and guests.  You will need to be a bit early to be in a position to greet all comers as they enter the room.  You should have the Badge box at hand so that you can give members their badge as you welcome them.  You will also be there to greet any visiting Rotarians or  Club Guests. You may also be asked by the Treasurer to collect the fees and tick the attendance sheet as well.
2.  Rotary Information.  This session is not held every week depending on the program.  When you get your Rotator on Sunday night, if you are nominated for this, you then have a couple of days to do some research.  All that is being looked for is a 2-3 minute talk about something interesting you have discovered about Rotary.  The idea of the segment is to improve Rotary awareness amongst all members.
3. Sergeant.  This is an important position that allows you to bang the gong.  The sergeant is the person to ensure that the meeting starts on time and keeps the meeting in order.