Eric had the correct response, what do you want to learn and why? How much theory do you know now? What instrument do you play? We could presume piano since you're posting here, but I don't want to make that assumption. How old are you and where are you in the educational process (what grade are you in)? What is your goal as a musician? The answers to these questions will help us guide you.
As a way to get started I see your options as:
1 Get a teacher, find a way to take a course in music theory (typically about two years). This is probably the best way to go, you get a real curriculum and have a teacher to correct your work.
2 Enroll in an online course, you'll get a real curriculum and can do it at your own pace, not sure how they handle the correct your work part.
3 Watch Youtube videos, you're dependent on the quality of the presenter and you won't know what you don't know until it's painfully obvious.
4 Buy a book, actually not a bad idea, but you'll have to treat it like you're taking a course and you won't have a teacher to correct your work.
There are probably more options, but you should understand that learning theory involves more than harmonic analysis. There's ear training, rhythm, form and structure. To me at my advanced age, learning theory was putting a name to a lot of things I already knew and adding ancillary skills that enhanced my ability to learn and also compose music. Still, there was a lot to learn in the real world, but knowing theory helped me organize my thoughts as I gained knowledge.