What have you tried?
A simple approach would be to (a) use a
DocumentBuilder to build a org.w3c.dom.Document instance from your file. Then you can (b) go through the document to find all the elements with the tag name "staff". (c) You are right: you can use getAttribute("id") to look at an element and see if it is the person you want. (d) Finally you can pull out specific pieces of information from that element's children:
NodeList nodeList = element.getElementsByTagName("firstName").item(0).getChildNodes();
Node node = (Node) nodeList.item(0);
String firstName = node.getNodeValue();
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I think it might be a good idea to create a simple class for an Employee (a better name than "Staff", I think) and from the document create a List of all the employees. This would help if the user wanted to make multiple searches.