This is primarily for my boys (14yo is currently interested in law, 10yo in marine biology). We use Khan Academy because that was what was freely available when I started looking into homeschooling resources.
First and foremost
Everything requires basic literacy and numeracy. So get through the high school maths on KA (start at your grade level, if it's too hard, play down, if it's too easy, play up, if you're in high school/over grade 8 and it's too hard, keep dropping til you can do it, then work back up).