Back when I first went to Khan, when I went into the math section it gave me an option to do a placement test.
If it's not giving you that option, just pick what level of math you feel comfortable with and start with that section, if it's difficult but doable, continue on from there. If it's far too difficult, back up to a prior section. Rinse, repeat.
In general - And, yes I know that some can be flip-flopped, but this is a general order which will work.
High school statistics