As a former child from the US, my only recommendation is not to take the US school system too seriously. Every state has a different system which is further broken down into subsystems where the quality of education can be completely different. 20 years ago the standard quality of math education was terrible where I grew up, I spent my whole life trying to find resources that were outside of the system. And when I was a teachers assistant during university I was often surprised how little math the students knew. That being said, find a textbook on algebra with a solutions manual and do all the exercises. If you can't figure out an exercise use online tutorials or lectures to make sure you understand all the concepts. If you can figure out all the rules and tricks of algebra you'll have a more solid foundation than a huge proportion of American university students and you'll be ready to apply them to the other fields of math you want to study.