Music theory is one of those difficult lessons that people find hard to grasp. It seems like the only people who are confident in music theory took piano lessons for 15 years or have a music degree from a university. The common person, however, often has a need to understand basic theory, but does not have the time or money to invest in a formal course.
Ironically, the most absolute and basic element of being able to study music theory is actually not considered music theory. It's called note reading. You will need to develop, at minimum, a basic to intermediate understanding of standard music notation in order to learn music theory on the internet. There are many options for studying basic note reading, and you can access software that will provide exercises on the internet, or just pick up a basic note reading book at a music shop. The best thing about using a computer to learn is that your mistakes will be identified right away, and the computer can show you how to fix them. Note reading will be the foundation upon which you build your music theory skills.
Once you have gained an understanding of basic to intermediate note reading, then you'll find that your quest to learn music theory on the internet will progress more smoothly. There are dozens of websites and program designed to help intermediate level learners understand music theory on the internet. Some are tailor to players of specific instruments or specific genres, and you'll have to choose what you'd like to focus on.
After note reading, the next thing you'll want to study is keys and key signatures. If you play an instrument, it may help you to learn some scales to help you understand and memorize the major and minor keys. Once you can understand keys and key signatures, you can move onto learning about chords, including their structure and what roles they play in music. You'll want to learn how to build all types of chords including majors, minors, finished, and seventh chords.
Believe it or not, there are some patterns in music that repeat themselves over and over again. Regardless of the genre, you will find certain chord patterns that appear across the board in every style of music. After you have mastered chord construction, then you will begin to study the order in which chords are commonly found. You can put this idea to practice by writing some songs of your own!
Depending on your instrument, there will be dozens of options for learning advanced theory. Guitar players will find that more advanced chord structures exist, and then study those structures and practice by using and finding these complex chords in music. All players will want to study modes as well, which is basically another way of forming scales outside of major and minor keys. If you are interested in writing music, you will also want to study more advanced uses of chords within music.
Music theory is a broad and dynamic concept to study. Some people spend their entire lives studying it, as there are PhD programs for music theory! You do not have to dedicate all of your time and energy to music theory in order to understand it. The course offers and materials that are found online make it possible to learn music theory on the internet.