I just finished high school and am going into a biology undergraduate degree, I'm getting into biochemistry too and would like to learn more about it through online platforms or even non-fiction books to use my time effectively this summer. So far I've been using online flash cards to learn more about the jargon and would like to hear some suggestions of platforms or books to read that teach concepts strongly tied to biochemistry beyond what a basic high school curriculum would teach.
There are several reference books for biochemistry, some more readable than others. I personally recommend two of them: (1) Lehninger et al. Principles of Biochemistry and (2) Voet, D., Voet, J., et al. Fundamentals of Biochemistry: Life at the molecular level. Both books contain the most important and basic features of each subject. The chapters are generally well written. I have to say that, from my own experience, I consider that Voet's book is more enjoyable, given that some of the sections of Lehninger's can be a bit tricky. Whichever book you choose, make sure you get relatively new editions. Good luck!
As a side note, even though biochemistry is one the first proper biology classes one takes in undergrad, there are some background requirements. Taking organic and inorganic chemistry courses before is important and some understanding of thermodynamics will certainly come in handy.