if:
the amount is in column B and
the checkbox is in column C and
the result is in column E
you can use this formula:
E2=if(C2, B2, 0)
this means... select cell E2, then type (or copy and paste from here) the formula:
=if(C2, B2, 0)
to fill this down, select cell E2, copy
select cells E2 thru the end of column E, paste