I have been playing clarinet for over 5 years and music is something that is deeply ingrained in my life. I am in high school and I am thinking I will most likely major in music (specifically go into music education) and I would like to get a head start in theory(because I am aware it is very challenging) as I would one day also like to be able to compose my own piece for band. The problem I have and ultimately my question is, where and how do I start? Would it be possible to learn everything online? (there is so much information online it is somewhat overwhelming) Any advice would be appreciated and I am serious about music and definitely willing to put in the effort.
With five years of playing under your belt, you probably know a fair bit of theory, but just aren't aware of it. Now's actually a good time to start; you'll get a lot of lightbulb moments - ah, that's why so and so works!
A teacher is an obvious good choice - books are o.k., but they won't answer a question there and then, unless it's already in the book, like a teacher should.
Another way is to use the exam boards - these vary from country to country - but the point here is they grade stuff, and work through sequentially. Books are available, and exams can be taken if you like, and there are inevitably test papers to check your progress, along with handbooks. In U.K. there's ABRSM., Trinity, LCM for starters. AB takes the traditional approach and comes from the classical standpoint; LCM has a modern approach, looking at theory in a rather different way.
It should go without saying that one of the best resources is this site!! Although each question is treated on its own merits, and there's no order to sieve through, there are plenty of erudite answers, most of which will give you lots of useful theory knowledge.
Bear in mind all along, though, that music theory is purely that. Ways trying to explain what happens, not sets of rules that must be obeyed at all times and costs! And also, try to make the theory practical - play the stuff on your instrument, make it come to life.
A teacher could be useful. If you have any problems, you can ask them, or if you complete a theory worksheet, they can correct it. I had to learn theory in 2 months because my father "accidentally" registered us for a theory test, so we bought the two theory books, and they can teach you without you needing a teacher. I'd recommend them because you can learn from the books, and there are some worksheets inside.
This should probably be a comment more than answer, but it's long:
I am aware it is very challenging
Relative to something like math or physics or computer science, it is not all that challenging. Anyone of 'average intelligence' can master the basics of music theory with a few months of application. Don't scare yourself before you start - never a good idea.
Advanced theory gets more challenging, certainly, and like any study, it can became arcane and difficult, but "it's not rocket science". Just proceed in an organized manner and take some time to think about things you don't understand right away. It's also a "living science" - constantly changing and evolving.
Arguably the most difficult thing about music theory is just that: Expanding and modifying it to accommodate and explain newly evolving forms of music. For example, 50 or 60 years ago, there really was no such thing as formalized "jazz theory", but today, you can get a degree in that. On this site we often see questions about how to explain the theory behind rock and pop music, and the answers can be elusive. Here's one that's fairly recent: Why does the chord progression (i-)#IV-i sound acceptable?
As for where to start on your own, if you've been playing clarinet for 5 years, you certainly know how to read music quite well and a good deal of theory, although maybe not all the formal terminology - that puts you way ahead of the game: Most elementary theory books devote a lot of time initially to teaching how to read and write music, and you should have most of that already.
Music Theory For Dummies is comprehensive and well written - it's modeled after a college music curriculum. They do spend time on reading music, but a review probably wouldn't hurt you and there may be new things there you didn't know. Then they go on to discuss a lot of very important and useful material.
I am mostly self taught - two college courses and some lessons from a jazz pro. So, I don't know all that much. I do know that Music Theory For Dummies is an accessible and valuable resource for me.
Another thing to look into is music classes in your area- maybe a local community college or something like that.
You can also try online resources, but make sure you use reliable ones, for example: Berklee Online is Berklee College of Music's Online School. (Unfortunately, they are not cheap.)
Beware of cheap/free lectures on youTube,etc, or taking upon yourself to read lots of articles in wikipedia. Virtually every day we encounter new people on this site who are almost hopelessly confused about music theory because all their study has been from free, online sources. (I can't help but wonder if it's such individuals who gave you the idea that "it is very challenging".)
The problem with doing that is not all the sources are reliable, and when you are a beginner you don't know how to be selective - you tend to absorb everything, good, bad or mediocre. More importantly, you generally don't learn in a progressive and comprehensive way - it's like a buffet that you pick and choose from. That's why you need good books and teachers - they teach in a proven, systematic and organized fashion.
Each of us learns in different ways and books and teachers teach in different ways. I had a teacher who taught me well, but she only taught what she thought was important. I found the need to search for answers in books also, and some of the books weren't able to explain in ways that made sense to me. It took persistent searching at the library and bookstores, but I found books and articles that helped me understand music better and filled in some of the holes in my education. I find that music study never needs to end and since I enjoy it very much, it's a very comforting thought