Where does one draw the line to determine what is ethical to share and what isn't? I am starting to design my personal website and I would like to devote part of the site to sharing knowledge in the fields of Mathematics and Computer Science. 

My main concern is determining what is ethical to share and what isn't.

I don't plan on publishing solutions to homework exercises or questions from my previous exams, as this seems obvious to me.

What I would like to share is the content of the individual subjects such as:


Definitions
Theorems
Examples (Those of which I create myself)
References
My personal interpretation of the subject matter


My concern is in regards to sharing definitions and theorems is ensuring I am not plagiarizing works from textbooks and professor's lectures.

Since there is only but so many ways to rephrase a definition before it loses its meaning, how much should I change the representation of a definition (or Theorem) so I am not plagiarizing how it appears in particular text?

My final concern is in regard to reference other sources of knowledge. Throughout my college experience, I am constantly looking for multiple resources to supplement my learning. For example, YouTube video series X does a really good job with an introduction to Linear Algebra. Would it be ethical to reference or add links on my personal website to YouTube video series X?
You can avoid plagiarism the same way you would in a research paper: by citing your sources! So long as you’re not just quoting the same sources over and over again, you can quote a short excerpt from a relevant source and cite it. It’s also worth noting that definitions of common terms, as well as equations, are not normally subject to copyright, because of the exact reason you mention: there are only so many legitimate ways to define a term or write an equation, which places a constraint on expressing them without using someone else’s definition.

As far as links go, there’s no reason you can’t cite what you want.