suppose that i have these values in column A
1,2,3,4,5,6,7,8,9,10,11
i need to sum  the even numbers  2,4,6,8,10 and so
thats it

There are at least 2 approaches.

One is

Put in another column ( say your numbers  are in A3 to A10) put in B3
=if(mod(A3,2)= 0,A3,0)
Copy this down to B10
Sum B3:B10

The other is to build a function which is given a range as a parameter and
steps through each cell in the range
Testing if it is even
Adding it if it is
Returning the total

Another way would be to use a conditional array...say you have values
in range a1:a12, you would enter the summation formula in a13 as follows:
=SUM(IF(MOD(A1:A12,2)=0,A1:A12))
to get the formula to work, you MUST hold down the <Ctrl><Shift> then
<Enter>. The result formula would have two brackets around the
formula like below...

{=SUM(IF(MOD(A1:A12,2)=0,A1:A12))}

the result should be 30

but i need to use one formula in one cell only to sum  all even
numbers in specific column