how do i create cmdlets that invokes a function

I have zero knowledge of widows powershell. I have situation where i need to create parameter and function(a wrapper). Function when invoked return string and set the value to one of the parameter. I don't even know where do i write powershell script and i think there is ISE where i can write but how to build them get the bat file?

My situation is something like this.

Command is ExportBill.exe which takes three arguements : filename, customerType and Id

function GenerateFileName(string fName)
{
return String.Format("{0}{1}",fName.ToUpper().Substring(0, 3),DateTime.Now.ToString("yyyyMMddHHmmss"));
}

ExportBill.exe -fileName GenerateFileName() -customerType "WholeSeller -@Id query_parameters

How do i do it in Powershell?