I considered giving the same advice as Subhotosh Khan, but there are some caveats.
One thing to be aware of with a placement test is that they will tend to place you in a class where you have a 50% chance of passing, or something like that. The placement recommendation will not reflect all the specific details that you do or do not know; you might be weak in a couple specific topics that are important, but be placed into a course that assumes you know them well. So if the results tell you such details, you might want to find books that review those weak topics first, before jumping into assembly-line education. (And if you do take courses, you should take full advantage of any tutoring they offer, which would be better able to fill in those gaps for you.)
Also, it's quite possible that courses there (even though they typically start with very basic elementary-level courses) will be designed to prepare you for subsequent courses, and may skip over some topics that you would wish they covered. For example, the community college where I teach and tutor doesn't have a course in high school geometry, but just touches on certain geometrical topics alongside arithmetic, algebra, and so on.
I also recommend focusing on an organized course (even if you do it yourself without a teacher) using a textbook, rather than using random videos. Khan academy may do fine, if you follow it as a course rather than just searching for videos on individual topics. I haven't looked at their scope to see if it would do everything you want. The important thing is that whatever you study should go from topic to topic in an orderly way, so that you can make sure you master one topic before moving on to others that depend on it.
I also agree that face-to-face courses, if possible, are valuable; online or computer-based courses can lack the human interface that most of us need, especially at the beginning.