How To Write Sum Formula For A Range In Last Empty Cell In Each Row

I have a spreadsheet. In this sheet the data is added daily so one column is increased everyday. The rows may also be increased. What I am trying to do is to sum the data in each row for a range of first cell in the row to the last non empty cell in that row and this has to be done for all the rows. So I thought the for loop would be useful for such requirement and I tried to write the following code. But unable to write the sum formula in the last empty cell in a row and I get the value error. The code is as follows.....

Sub sum_on_LastEmptyCell()
'find the last empty row in column A
lrow = ThisWorkbook.Sheets("sheet1").Cells(Rows.Count, 1).End(xlUp).Row + 1
'find the last empty column in a row

[Code] ........

Though one of my friend told that it can be done with "with and end with block but I am not aware of with and end with block.

It would be better if you tell me that how can I put the sum formula in my code. How can I use variables which return the row number and the column number in the sum range within the For loop because I want to put If Else condition for the calculations within the For loop i.e. if certain condition is true then I want this calculation to take place otherwise do something else. Moreover it will be easier for me to understand. Because I can use different formula based on different cells as well. Is there any way to do this?