You'll have to use an index number, to indicate which one of the found elements with that class you want to target. This will select the first one:

document.getElementById('theId').getElementsByClassName('theClass')[0];

I've never figured out how to select all classes with getElementsByClassName, so that's why jQuery is so great.

What I use instead is the tag name, because in most cases (at least for me) the tags for the class are all the same:

var tags = getElementsByTagName('a');
for(i=0;i<tags.length;i++){
	 if(tags[i].className==='theClassWanted') {
	 	 //code to execute
	 }
}