Welcome to the tagging team! You may or may not have read this, so I will post the link: http://imslp.org/wiki/IMSLP:Tagging
It gives a good overview of the tagging process, but is an awful lot of information to absorb all at once.
I also just want to add to Kenny's comments about compilations, that there are quite a lot of compilations that are due to be split apart so that the individual pieces can be on their own page under their composer's work lists. These have a notice at the top of them saying something to the effect that the work is due to be split up.
Don't waste your time tagging these -- it inevitably involves downloading them, seeing what's in them, and then listing all the tags, which is a waste of time if the page is going to be split up anyway. Also, leaving the page on the list of untagged works is at least a reminder that it needs to be split.