http://www.emusictheory.com/practice.html
This website has a whole bunch of interactive activities/games for you to learn music theory and actually have fun with it!
http://teoria.com/tutorials/
This website provides very in depth descriptions of music theory and it even teaches you music notation!
http://www.dolmetsch.com/introduction.htm
This website presents information about music theory in a "forum" or FAQ style.
Another big important thing involving music is copywriting your work. If you work really hard on a song wouldn't you want anyone to steal it, right? Well if you want your work protected UNDER LAW, then you need to copyright it! These are two websites that are very informational on this topic.