This formula  is very simple.
Since I don't know what is in your cell C2,
start C3 with the formula:
=B3
then, in C4 put:
=C3+B4
copy this formula to the rest of the cells in column  C.
(It'll automatically change to "=C4+B5, =B5+C6... etc)
Now, if the B cell is blank, maybe you want the C cell blank also.
Try:

=IF(B4<>"",C3+B4,"")

in C4...