This is a bit dated, but it is still effective as a way to understand why you might want to learn markup languages
This book is available for free to UNC students through Safari Books Online, but it is a great reference tool and should be part of your professional bookshelf.
You don't have to watch it, but this introduction to HTML might be instructive.
Similarly, you don't have to watch it, but this introduction to XML, by the same presenter, might also be instructive.
Don't think you have to know or remember everything in both readings.
They are to introduce you to the two markup languages
If you want more on XML, you may find it useful to read XML Foundations, pages 42-72 in Robert J. Glushko's, Document engineering : analyzing and designing documents for business informatics & Web services, Cambridge, Massachusetts: MIT Press, 2005
This is not a required reading. It is optional, but perhaps is one you might want to keep for later reading.