There are a large number of music theory tutorials available on the internet, mostly on YouTube. One of the best I've found is the series of videos by Dave Conservatoire.
This is a comprehensive music theory course divided into numerous 4 to 15 minute videos. It starts out with the absolute basics, and progresses into more advanced theory. The explanations are remarkably clear. I think this would be a good site both for complete beginners and for people who know some music theory but want a review.
Most of these videos seem to be also on YouTube, but it's more convenient to work through them on the Dave Conservatoire web site. The site is free, though they accept donations.