The last time I studied mathematics and algebra was six years ago. I've since forgotten everything except quadratic equations, trigonometric functions and other things at the same level. Now I want to relearn all this: combinatorics, linear algebra, probability theory. I'm looking at articles in wikipedia, and don't understand anything except the basics. Trying to understand a topic, I dig deeper and deeper into wikipedia and after a while, I can't even remember where I started.
So my question: what things I must learn for first steps?