How to Start Learning Math? I am grade 11 student who is weak in math. I'm been weak in it since grade 7 and now I've decided I want to rebuild basic math knowledge. How should I getting start?
I know I am weak in problem solving and critical thinking skills. But more specifically I don't quite have a grasp on functions so I think that's where I'd like to start.
How can I effectively teach myself high school level math including the above thinking skills and specifically functions using online resources?
 A: I've decided to turn my comments into an answer.  For anyone wondering, that's why it looks like OP is having an imaginary conversation in the comment section. ;)
 General Advice 
As with any skill you want to get better in, the most important thing is $$\text{practice, practice, practice.}$$ Ideally, you'll want to pick up a math book that interests you and spend as much time as is reasonable every single day working through it. You'll find that your math skills will be measurably better within a few months (which I know seems like a long time, but that's how these things go -- Rome wasn't build in a day).

 Suggested Resources for Learning Basic Algebra Including Functions 
I personally don't use too many online resources for learning. I tend to prefer textbooks. A textbook I can recommend is called Precalculus by Stitz and Zeager. It's free and open source. Don't be too scared of the fact that it's a precalculus book, it doesn't assume you know too much. You may want to start by reading through the separate "Chapter 0" file first, but the part on functions is in Chapter 1.  Try looking looking through it and see if that answers some of your questions about functions.
If you'd prefer an online video lecture, I don't really use it myself, but I know people who really like Khan Academy. You could try their section on functions. Caution: IDK if they have exercises for you to work, though. If not, make sure you find exercises somewhere else to work through. In theory, you should be spending more time working exercises each day than learning new material. That's the way you'll get the deepest understanding.

 More Resources 
The way to really learn mathematics is to start reading interesting books on it for fun.  Unfortunately, most of the interesting books aren't going to be much help to you if you don't have your basics down, yet.  Nonetheless, I'll give a couple of recommendations for fun stuff you can learn on your own once you've gotten comfortable with your basic algebra and geometry stuff.
The New Mathematical Library is a bunch of books that were published during the "New Math" era that started around the 60s.  Each book is aimed at roughly the level of high schooler and includes a bunch of info that is usually not taught in US schools.  There are some really interesting volumes including Graphs and Their Uses, First Concepts of Topology, and each of the geometry books by Yaglom.  Find one on a subject that interests you and see what you think.  They're all relatively short so you should be able to get through a whole book in a couple of months if you work at it.
The Art of Problem Solving books are at a little bit higher level than most middle/ high school level books and are a fun way to challenge yourself.  Plus they include a couple of topics not always given as courses at school like Probability and Number Theory.  But, more useful than AoPS's books is its community.  It can be quite valuable as a resource for finding and discussing interesting problems with your peers.
