If you are working with XML files created on different platforms, note that the End-of-Line (EoL) character differs between Windows (CRLF) and Unix (LF). For more information, see the Wikipedia page on Linebreak at http://wikipedia.org/wiki/Linebreak.
You can find more information about XML from multiple sources on the Web. http://wikipedia.org/wiki/XML contains an introduction to XML. XML functionality is available in most of the Integrated Development Environments (IDEs). You can also find a wide variety of free XML editors with XML validation.