Really, that is going to depend on wht you have in your file, and what you want to do with it. But...
string text = File.ReadAllText(@"D:\Temp\myFile.txt");
XmlDocument doc = new XmlDocument();
XmlDeclaration dec = doc.CreateXmlDeclaration("1.0", null, null);
doc.AppendChild(dec);
XmlElement root = doc.CreateElement("MyRoot");
doc.AppendChild(root);
XmlElement elem = doc.CreateElement("MyElement");
elem.SetAttribute("MyData", text);
root.AppendChild(elem);
File.WriteAllText(@"D:\Temp\myFile.xml", doc.OuterXml);