Where should I start?

If you're an adult who hasn't done mathematics in quite some time, I recommend doing the Khan Academy course challenges, or Math Mammoth End of Year Tests, for each grade level, to help you identify where your knowledge is lacking. While doing the course challenges, note any questions that came to mind, or problems which you had no idea how to solve. Then you can revisit this site to fix the holes in your knowledge before attempting harder topics.

Khan Academy course challenges

Math Mammoth End of Year Tests