How do I navigate to next row?

I have a spreadsheet with the columns going to AP and the rows going to 140. I need to change the value in some cells. I want to be able to tab (or arrow) over to the adjoining cell and when I get to the last cell in the row, I want to hit Enter to start in Column A of the next row.

I can't find an easy/quick way to get back to the first cell in the next row. If I press Tab, it adds another column which then I have to delete. If I hit enter, I go down one row in the same column. If I press Home, I then have to use my mouse to select the first cell or I bounce back to the last cell.

What am I missing here? I just want to navigate through the row and then begin in the first cell of the next row. But, I can't figure out how to do this!!

Also, is there any way to make it stop adding columns (when I press Tab)?

Does anyone know? Thanks.