store data from richtextbox to xml file

Well, assuming it's well formed XML, you could save the text contents of the RTB to an XmlDocument and, if you wish, save it from there to a file on disk. Here's some basic code for that:

Dim doc As New XmlDocument();
Try
   doc.LoadXml(richTextBox1.Text)
   doc.PreserveWhitespace = True
   doc.Save("c:\myfiles\myfile.xml") 'or whatever
Catch
   MessageBox.Show("Invalid Xml")
End Try

Notice that the Save() method will overwrite any existing file with the same name.

I'm not sure what you mean by 'start the Xml' but you could reload it into an XmlDocument using:

doc.Load("c:\myfiles\myfile.xml") 'or whatever

If you have .NET 2.0 or later, you can also use the RTB's SaveFile() method to save the XML directly to a file as plain text:

richTextBox1.SaveFile("somefile.xml", RichTextBoxStreamType.PlainText)

Again this overwrites any existing file of the same name. It can be reloaded into the RTB for further editing using the LoadFile() method:

 richTextBox1.LoadFile("somefile.xml", RichTextBoxStreamType.PlainText)