A subreddit for people who care about composition, cognition, harmony, scales, counterpoint, melody, logic, math, structure, notation, and also the overall history and appreciation of music.
Try checking out the beginner's resources listed below if you are looking for exercises, websites, or books!
1. Maintain semi-professional discourse. That is, casual language (mild profanity) is okay, but refrain from taking an overly adversarial tone. Along the same lines, don't just tell someone to Google the answer. There are multiple reasons why someone might prefer to get an answer from reddit instead.
2. No homework help on specific assignments. It is against the Academic Honesty Policy of most schools and courses. Our subscribers generally dislike this kind of behavior. Please ask your IRL teacher/tutor for homework help instead.
3. No memes, image macros, rage comics, Facebook screenshots, or other similar material. They are not conducive to the informative atmosphere we'd like to maintain here. Post them in /r/classicalmemes instead.
Both musictheory.net and teoria.com offer basic lessons in music theory, but more importantly, they have great drills for you to practice chord identification, aural skills, etc.!
Convenient, one-page summaries of just about every music theory topic you might come across in freshman or sophomore theory!
Answers the question "What is a good book to learn music theory from?"
A Khan Academy styled music theory site—lots of helpful video demonstrations.
Please know that Wikipedia is especially bad for music theory topics. The above-listed resources are a thousand times more reliable!
Other (formal music education, but not a professional musician)
Professional musician (non-theorist)