Help with Excel

Hello,
I’m creating a template in excel. In column A i have the month (Jan, Feb…) and in Column B the expenditure. But I still cannot find a formula to sum in a new cell the expenses that correspond only to January, and in another cell just the expenses of february and so on. I have something like:
=if(a:a=”JAN”, SUM THE CELLS IN COLUMN B WHICH CORRESPOND TO JANUARY….
Everything is in the same sheet.
Any idea???????

Thanks!!!!!