I am an undergraduate non-math major. I just finished my university's engineering calculus series, looking forward to linear algebra in the coming semester. To be frank, I always despised math because I couldn't help but find it tedious. I have recently had a change in heart, however, due to advice and encouragement from my favorite professor and some experience with proving logic statements (which I mysteriously find ridiculously fun...). Starting to read math review journals sounds like a great way to begin as an amateur mathematician.
So where can one find (free, online) mathematical articles with a fighting chance to be understood by high school students and undergraduates?
(The "free, online" part of the question is nonmandatory. I have access to academic journals through my university.)