Hi,
If you are new to XML I would advise you to start with
www.w3schools.com tutorials on XML. They are very simple and straightforward. Next would be any good XML book.(if you are aiming for certification then might as well read Professional XML 2nd Edition). The online tutorials on xfront and IBM are also good.
I took almost all the mocks exams mentioned on the javaranch xmlexam links page.I scored between 70-80 % in the exams. IBM mock exam score was 72%, since I finished it in a hurry (20 mins) and made some silly mistakes.
For practice, download XMLSpy trial version .
All the best
-Ujwala