XML is a syntax, which means a simple text editor is enough to get started.
However, having an XML-specific editor that shows you when and where your document is not well-formed is almost indispensable for productivity. Such editors may also allow you to validate XML documents against an XML Schema, or even generate XML Schemas from XML documents (and vice versa).
Some examples of editors are oXygen, Atom, Eclipse and Altova XMLSpy. An alternate solution is to use a command-line XML parser such as Apache Xerces.