If you mean a file that notepad can read, then you mean a text file: i.e. a file with teh extension ".TXT"
.TXT does not have a concept of fixed widths, so if you want to lay out the file content then you need to write to teh file in a structured manner. Fortunately, .NET provides
string.Format
which can do this, as well as the format string parameter to teh Write and WriteLine methods.
What you need to provide in the format string is the complete fixed width format that you want to output - which will vary depending on your data types.
For example, if you want to output an int "i" and a string "s" as fixed format data:
int i = 12;
string s = "ABC";
myStream.WriteLine("{0,6} {1,10}", i, s);