So assuming you are starting in row 1, you can enter a formula in E1 that says "=C1-D1", which will show you C1 minus D1 in E1. Then if you hover your mouse along the bottom edge of E1, you'll see a yellow dot appear. Grab the yellow dot and pull it down. This will copy the formula from E1 into each cell of E that you pull the handle down over. E2 will become C2-D2, E3 will become C3-D3, etc.
EDIT: As I read over your question again, I think I misunderstood a bit. Is what you want, for example, E3 to reflect the sum of all incomings and outgoings from row 1 through 3, and then E4 to reflect all incoming and outgoing from row 1 through 4, etc.?
EDIT2: Ok, got it. Same assumption that your Incoming column C is positive numbers, and your Outgoing column D is also positive numbers, you can write the following formula in E1:
Code:
=SUM($C$1:C1)−SUM($D$1:D1)
As you pull the yellow handle on E1 and copy that formula down the column, the $ signs hold the variables after them constant. So once you copy this formula down, E2 will say:
Code:
=SUM($C$1:C2)−SUM($D$1:D2)
And E3 will say
Code:
=SUM($C$1:C3)−SUM($D$1:D3)
And so on. So it adds up all the Incomings from the top down to your current cell, then adds up all the Outgoings from the top down to your current cell, and subtracts Outgoings from Incomings.