Music theory is the study of how music works, the building blocks of music, and what is needed to understand music notation. For the beginner, music theory is the study and descriptions of music rudiments required to notate and read music, such as key signatures, time signatures, rhythmic notation, pitches (notes), scales, modes, chords, tensions, and elements of composition such as rhythm, harmony, melody, and counterpoint.
There is no single definition that describes all the parts of music theory. As a practical discipline, music theory is concerned with the fundamentals of how music is made, and also with analysis of sounds, including silence, and how they relate to music.
The internet is full of materials and there's definitely a lot about music theory that you can learn on your own. There are many good internet resources for the basics of music theory, but unfortunately there are fewer good online resources beyond the basics. When you want to develop your music theory knowledge, lessons with a tutor will always be helpful.
A private tutor can help you decide what lessons will be useful for you personally. Many internet resources are inaccurate and provide wrong information. And this information is too broad. There are different lessons that a tutor will give to a classical music lover and a metal guitar player.
Also, a teacher can help you make sense of conflicting information. You may notice that people disagree about music theory a lot. Or you might get confused because everybody uses different terms for the same ideas and sometimes the same term for different ideas! Good teachers spend time thinking about which perspective to introduce to their students first: they make sure that what you learn is consistent. Once you've learned one perspective, it's much easier to understand how a different perspective relates to the first one. It's hard to learn the concepts and sort out the conflicts at the same time, especially without a guide.
Many people say that music theory is hard, but this concept is wrong. Music theory is not "hard" rather it is certainly "complex". Sure, there are many things that you need to understand when learning music theory, but each single one of these things is easy, and you do not need to understand the whole thing before you can use it. You can learn it one piece at a time.
Like any other theory in music theory there are many concepts that you need to learn. It may seem impossible to remember everything at first but didn't the multiplication table seemed impossible at the beginning too? Music theory is difficult to grasp because it uses visual methods to describe what we hear. That's a big problem for music theorists because our brains interpret music hundreds of times faster than it takes to explain it. This is why music is powerful. It's also why music theory seems hard to understand.