If you want the number for the current month, use Month ( Get ( currentDate ) ). If you want the name of the current month, use MonthName ( get ( CurrentDate ) ).
You may also find one of these calculations in a calculation set to return date useful:
DateField - Day ( DateField ) + 1
Date ( Month ( Datefield ) ; 1 ; Year ( Datefield ) )
Both return the date of the first day of the same month and year as the date in DateField and such a field can be useful for searches, sorts and as a relationship key for working with all records of the same month and year.