I would certainly recommend Ardalambion (
http://www.uib.no/people/hnohf/)! It has thorough information about the languages Tolkien created. A basic knowledge of a language is needed to be able to work with a dictionary (also provided on that site).
Example:
barad means "tower", but in plural it becomes
beraid, which you probably won't find in a dictionary. Next to the rules of plural-formation, there's the rules of mutation, which may alter consonants. A basic understanding is required to put a dictionary to its use.
If you have some linguistical knowledge, I would recommend the courses that are provided on that site. Both the Quenya and the Sindarin course are not easy, but provide a lot of insight.