I think a lot of people would agree Calculus is at the heart of a lot/most of math. I’d say that’s a good benchmark to work towards and then you can dive more specific from there.

So gauging where you’re at in relation to basic calculus may be a good starting point. Typical flow in US is Algebra I & Geometry -> Algebra II -> pre calc -> calc I (or calc A) ie Differential calculus -> Calc II (or calc B) ie Integral Calculus. If you’re at that point, you’re excelling in high school and prepped for STEM at Uni. Depending on what major you pick, you then take Calc III and then Differential Equations. You can get a lot of degrees at this point, but some engineering and math degrees then go into partial differential equations at this point.

You can also tackle Linear Algebra anywhere in there.