This ^. If you wonder, 'what complex numbers are all about', then chances are you can't give the universal property of the tensor product, or the definition of

This ^. If you wonder, 'what complex numbers are all about', then chances are you can't give the universal property of the tensor product, or the definition of a Riemannian manifold, or a complex Lie group, either. I suggest to look at the literature listings of any undergrad math curriculum to get suggestions on what to start with. There are reasons 'linear algebra' is usually taught before 'Applications of the representation theory of Lie groups in special relativity'. Or maybe I'm too conservative in my view on math education?