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