As long as you have good grasp of schema and entities in XML you are ok.
the stuff to learn about is actual prorgrammatic implemetation ,
If you are interested in using DOM or SAX API's well thats where the learning curve may be..
There is a book by deitel and deitel "How to program XML" I hope I am write,its been a while that I used XML.
It has questions and answers ( I know it did)