I find that the easiest way to get started with XMLA is to look at examples and the easiest way to get some examples is to use Management Studio. Most (maybe even all) of the activities that you can do by right clicking have the option to script the XMLA for the task you are trying to perform.
e.g. to backup a database, I right clicked, selected backup, de-selected "Encrypt back file", scripted the result which looked a bit like
<DatabaseID>Your database name</DatabaseID>
<File>Your back filename.abf</File>
Your next best resource is to use Books Online which you should have installed onto your workstation. Search for XMLA in the index and start reading.