I want to learn math! What should I read to advance my skill?

You should pick up a first-year university book in an elementary topic. "Elementary" here means that it doesn't assume any particular knowledge from you beforehand, not necessarily that it's easy. Some such topics are linear algebra, calculus, abstract algebra, discrete mathematics, elementary number theory and graph theory. Most other topics will assume you already know linear algebra and calculus.

I would suggest the book Discrete Mathematics by Biggs. It has a very clear style, and it goes through a lot of interesting topics in set theory, elementary number theory, abstract algebra, algorithms, graph theory, combinatorics, and so on. I throroughly enjoyed this book when I read it. My one complaint is it doesn't go very deeply into any topic, but it will give you a taste of a lot of things.

Some further advice: Pick only one topic and one book at a time, and focus on that. Make sure you learn from a book and not from some mix of sources online: a book will be much better organized to learn from than clicking links on Wikipedia. Read slowly and read every page from the start. Don't be discouraged if progress is slow: taking one hour to read one page is not uncommon. Do as many exercises as you can. Once you have a solution or a proof of an exercise, write it out, even if you think it seems too easy or silly. It will make you think through your arguments better.

I like the Art of Problem Solving books that do a nice job of teaching problem-solving technique. Here is a link to them.