I have an excel sheet with banking information (date, amount, category .... )
category contains (food, transportation, entertainment ... )
how do i find how much i spend on food between a timeperiod

Look under subtotals that should help

subtotal will not give me conditional sum. What I need to find is sum of all money spend on food from may 1 to may 31 OR sum of all money spend on transportation from June 1 to June 31 and so on

Assume dates in column A, Identity in column B and totals in column C. You can use SUMPRODUCT:

=SUMPRODUCT(($A$1:$A$20>=5/1/2007)*($A$1:$A$20<=5/31/2007)*($B$1:$B$20="Food"),$C$1:$C$20)

Adjust ranges as needed. Also you may need to work with date formats. It might be easier to set up start and end dates in two cells (i.e. D1 is start date, then D2 is End Date), and category in D3, then your formula would be:

=SUMPRODUCT(($A$1:$A$20>=$D$1)*($A$1:$A$20<=$D$2)* ($B$1:$B$20=$D$3),$C$1:$C$20)

Or you could use a Pivot table.