Trouble understanding schema xml in c#

I think I've been avoiding this as a fundamental in transitioning to .net, but I'm having difficulty reading and sorting XML documents, and understanding what seem to be key pieces of OOP.

I have an XML document that looks like this:

<?xml version="1.0" encoding="utf-8"?><ATX xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><section1s><section1 xsi:attr1="do" attr2="re" attr3="me"><section1 attr2="re" attr3="fa"><section1 attr2="re" attr3="fa">
      ...</section1>
    ...</section1s><section2s><section2 attr1="do" attr2="re" attr3="me"><child1 xsi:attr1="la" attr2="re" attr3="ti" /><child1 xsi:attr1="la" attr2="re" attr3="ti" />
      ...</section2>
    ...</section2s>
  ...

I see a 3D datagrid, with varying dimensions, titles and unknown depth. I don't understand the fundamentals of how to query this and return a value, ie

select section5.child3.attr2 from section5s where section5.child3.attr3 == 'ti'

So I'm trying to understand how to read the given xml schema

http://www.w3.org/2001/XMLSchema

in order to give me some clue, but I'm getting a lot of "Validating against xml schemas" and I understand more that it's defining an object, but I'm really just trying to get a single value from a file based on a specific condition.