column a .....date
column b .....name
column c .....amount
column d .....balance

start in row 1 with headings
row 2 with the opening balance in D2
enter in D3 =if(C3="","",D2-C3) this means in plain language

if cell c3 is empty show d3 to be empty, otherwise is c3 has a value
show the baance in D3

hover over the right hand bottom corner of cell D3
the mouse pointer will change to a "+" sign
left click and drag down as far as you want to go.

hope that helps