I need help writing a letter requesting to change shifts at my job...?

I just started working at a correctional facility and I'm clueless as where to begin with this request. I'm currently working the 1st watch (1045pm to 0700am) but I would rather work 2nd or 3rd watch. We get a month of training before we know what watch we will actually be on, but I was told to put in my request so I wouldn't be stuck with a watch I don't want. Could someone please help me or at least get me started? Thanks in advance!