During high school, I really fell off mathematics due to a lot of personal stress and the inability to focus due to bad eye-sight. This is very unforturnate, because I know that I am somewhat good at maths and I am very interested in maths. I know for a fact that I am very good at high-school level algebra and (maybe) derivations as well, but some areas I have always struggled with is geometry.
Are there any good books/online resources for an interested person like me? Where do I know where to start? Are there any good placement tests for mathematics that I perhaps can try to take to find out what level of mathematics I am currently at?
General Resources
Khan Academy. Definitely Khan Academy. Khan Academy has so much information about math - they have videos ranging from kindergarten level math to linear algebra and differential equations (and, needless to say, they have geometry right in the middle). But, even better, it's not just videos. Khan Academy has "missions" where you work through practices, and the more you do problems correctly, the closer they get to "mastered" and your mastery of the course increases in percentage. An image of what your mission looks like is below.
As a bonus, Khan Academy also has information about stuff ranging from computer programming to economics to science.
I would also suggest MIT's online courses on mathematics. It has many courses, along with online textbooks and textbook recommendations, notes, and much more.
Then, here is a site with a bunch of tests for different sections of highschool mathematics; you could test yourself and see where you fall.
Calculus Resources
I would suggest this book, Calculus Made Easy by Silvanus P. Thomson. I assumed I'd be in over my head with a book about calculus (I haven't had Algebra II yet) but I was pleasantly surprised with clear explanation and problems throughout to do. (Also, I'm pretty sure this book is a classic - yet another reason to read it.)
This site has not only resources on teaching yourself calculus - think worked-out problems, animations, excerpts from books, sample tests, and more - but it also has links to other calculus websites, categorized by what they've got. Categories range from tutorials to tests to demonstrations to practice problems (and, of course, a few fun links).
This site is basically the notes from a teacher's calculus course (and algebra and trigonometry), but with a few extra additions. For example, included are downloads of textbooks, review pages, cheat sheets, and even a section about how to study math. The site also has links to other helpful sites, like math help sites, helpful software, practice problems, and calculators.
Geometry
I'll be adding more suggestions as I find more information - hope this helps!