I'd recommend Khan Academy. As others have said, make sure you are very comfortable with algebra, geometry, trigonometry, surds and the like. After your fundamentals are strong, I'd recommend looking at their introductory calculus playlist, as that is one of the first things you'll learn. It is quite a steep learning curve, but it will be worth it. Other topics to look at include logarithms, polynomials and more advanced trig (like trigonometric identities).