This is the most effective route. BetterExplained can be a good supplement. Make sure you know how to learn effectively: check out the Learning How to Learn co

This is the most effective route. BetterExplained can be a good supplement. Make sure you know how to learn effectively: check out the Learning How to Learn course on Coursera. If you want to go even deeper than Khan Academy, the Art of Problem Solving books are great.