Khan Academy. Although mainly aimed at secondary topics, many subjects, such as mathematics, start at the very beginning with basic arithmetic.
Every topic has a short video lecture and a comments section to clarify any confusing points. Although the videos can be watched without logging in, it really is worth creating an account as you can keep track of your progress with the Knowledge Map. This shows you all the topics you have completed, those that you need to go back to and those new topics that follow logically from those you have already done. Khan Academy is now huge and the Knowledge Map will help you through the forest.
Although Khan Academy started with mainly Mathematics and Physics, it has grown to include other Sciences, Economics, Philosophy and more. It is also interesting to learn subjects that will probably never be done at school, such as philosophy and computer programming.
I also urged my students not to be in awe of content that was designed for older students. Once you have reached a good understanding of English and a mastery of basic maths, the rest is just new words and new symbols! If you end up watching something that is totally incomprehensible, just use the Knowledge Map to take a step back to a previous topic.
When I was a young student, I would often pick up books that were obviously way above my level. I didn't expect to understand them all, but they gave me a good idea of what was coming up on the horizon if I chose to go in that direction. Now we have the internet, but the aim remains the same - look into the future to find something that really excites you to learn more.