If we could only recommend one resource, this would be it.
Khan Academy is an American non-profit educational organization created in 2006 by Sal Khan. Its goal is creating a set of online tools that help educate students. The organization produces short lessons in the form of videos. Its website also includes supplementary practice exercises and materials for educators.
Beyond math, you can learn just about anything that interests you here. Explore, brush up on math you think you may need, and follow the progression Khan recommends.