Typically you would use dataFile.print(value); to write some text into the file. Use dataFile.println(value); for the last value on the line. If you separate the values with commas (dataFile.print(".")
you can read the lines of values into a spreadsheet by giving the file extension '.csv' (Comma Separated Values. If your data might include strings that contain commas, put double-quotes before and after the string so the spreadsheet will know that comma is not a separator.
What kind of stuff did you want to log?